From 62238c68867816a2e46c84bd87fc2ad728e47093 Mon Sep 17 00:00:00 2001
From: Marcus Pasell <3690498+rickyrombo@users.noreply.github.com>
Date: Fri, 17 May 2024 08:22:32 -0700
Subject: [PATCH] [typescript-fetch] Make instanceOf infer type and check for
 undefineds (#18694)

---
 .../typescript-fetch/modelGeneric.mustache    |  4 +--
 .../builds/allOf-nullable/models/Club.ts      |  2 +-
 .../builds/allOf-nullable/models/Owner.ts     |  2 +-
 .../builds/allOf-readonly/models/Club.ts      |  2 +-
 .../builds/allOf-readonly/models/Owner.ts     |  2 +-
 .../models/AdditionalPropertiesClass.ts       |  2 +-
 .../default-v3.0/models/AllOfWithSingleRef.ts |  2 +-
 .../builds/default-v3.0/models/Animal.ts      |  4 +--
 .../models/ArrayOfArrayOfNumberOnly.ts        |  2 +-
 .../default-v3.0/models/ArrayOfNumberOnly.ts  |  2 +-
 .../builds/default-v3.0/models/ArrayTest.ts   |  2 +-
 .../default-v3.0/models/Capitalization.ts     |  2 +-
 .../builds/default-v3.0/models/Cat.ts         |  2 +-
 .../builds/default-v3.0/models/Category.ts    |  4 +--
 .../default-v3.0/models/ChildWithNullable.ts  |  2 +-
 .../builds/default-v3.0/models/ClassModel.ts  |  2 +-
 .../builds/default-v3.0/models/Client.ts      |  2 +-
 .../default-v3.0/models/DeprecatedObject.ts   |  2 +-
 .../builds/default-v3.0/models/Dog.ts         |  2 +-
 .../builds/default-v3.0/models/EnumArrays.ts  |  2 +-
 .../builds/default-v3.0/models/EnumTest.ts    |  4 +--
 .../models/FakeBigDecimalMap200Response.ts    |  2 +-
 .../models/FileSchemaTestClass.ts             |  2 +-
 .../builds/default-v3.0/models/Foo.ts         |  2 +-
 .../models/FooGetDefaultResponse.ts           |  2 +-
 .../builds/default-v3.0/models/FormatTest.ts  | 10 +++----
 .../default-v3.0/models/HasOnlyReadOnly.ts    |  2 +-
 .../default-v3.0/models/HealthCheckResult.ts  |  2 +-
 .../builds/default-v3.0/models/List.ts        |  2 +-
 .../builds/default-v3.0/models/MapTest.ts     |  2 +-
 ...dPropertiesAndAdditionalPropertiesClass.ts |  2 +-
 .../default-v3.0/models/Model200Response.ts   |  2 +-
 .../default-v3.0/models/ModelApiResponse.ts   |  2 +-
 .../builds/default-v3.0/models/ModelFile.ts   |  2 +-
 .../builds/default-v3.0/models/Name.ts        |  4 +--
 .../default-v3.0/models/NullableClass.ts      |  2 +-
 .../builds/default-v3.0/models/NumberOnly.ts  |  2 +-
 .../models/ObjectWithDeprecatedFields.ts      |  2 +-
 .../builds/default-v3.0/models/Order.ts       |  2 +-
 .../default-v3.0/models/OuterComposite.ts     |  2 +-
 .../models/OuterObjectWithEnumProperty.ts     |  4 +--
 .../default-v3.0/models/ParentWithNullable.ts |  2 +-
 .../builds/default-v3.0/models/Pet.ts         |  6 ++--
 .../default-v3.0/models/ReadOnlyFirst.ts      |  2 +-
 .../builds/default-v3.0/models/Return.ts      |  2 +-
 .../default-v3.0/models/SpecialModelName.ts   |  2 +-
 .../builds/default-v3.0/models/Tag.ts         |  2 +-
 ...lineFreeformAdditionalPropertiesRequest.ts |  2 +-
 .../builds/default-v3.0/models/User.ts        |  2 +-
 .../builds/default/models/Category.ts         |  2 +-
 .../builds/default/models/ModelApiResponse.ts |  2 +-
 .../builds/default/models/Order.ts            |  2 +-
 .../builds/default/models/Pet.ts              |  6 ++--
 .../builds/default/models/Tag.ts              |  2 +-
 .../builds/default/models/User.ts             |  2 +-
 .../builds/enum/models/EnumPatternObject.ts   |  2 +-
 .../FakeEnumRequestGetInline200Response.ts    |  2 +-
 .../builds/es6-target/src/models/Category.ts  |  2 +-
 .../es6-target/src/models/ModelApiResponse.ts |  2 +-
 .../builds/es6-target/src/models/Order.ts     |  2 +-
 .../builds/es6-target/src/models/Pet.ts       |  6 ++--
 .../builds/es6-target/src/models/Tag.ts       |  2 +-
 .../builds/es6-target/src/models/User.ts      |  2 +-
 .../multiple-parameters/models/Category.ts    |  2 +-
 .../models/ModelApiResponse.ts                |  2 +-
 .../multiple-parameters/models/Order.ts       |  2 +-
 .../builds/multiple-parameters/models/Pet.ts  |  6 ++--
 .../builds/multiple-parameters/models/Tag.ts  |  2 +-
 .../builds/multiple-parameters/models/User.ts |  2 +-
 .../src/models/Category.ts                    |  2 +-
 .../src/models/ModelApiResponse.ts            |  2 +-
 .../src/models/Order.ts                       |  2 +-
 .../src/models/Pet.ts                         |  6 ++--
 .../src/models/Tag.ts                         |  2 +-
 .../src/models/User.ts                        |  2 +-
 .../sagas-and-records/src/models/Category.ts  |  2 +-
 .../src/models/DefaultMetaOnlyResponse.ts     |  4 +--
 .../src/models/FindPetsByStatusResponse.ts    |  4 +--
 .../src/models/FindPetsByUserResponse.ts      |  4 +--
 .../models/GetBehaviorPermissionsResponse.ts  |  4 +--
 .../src/models/GetBehaviorTypeResponse.ts     |  4 +--
 .../src/models/GetMatchingPartsResponse.ts    |  4 +--
 .../src/models/GetPetPartTypeResponse.ts      |  4 +--
 .../sagas-and-records/src/models/ItemId.ts    |  6 ++--
 .../src/models/MatchingParts.ts               |  6 ++--
 .../src/models/ModelApiResponse.ts            |  2 +-
 .../src/models/ModelError.ts                  |  4 +--
 .../sagas-and-records/src/models/Order.ts     |  2 +-
 .../sagas-and-records/src/models/Part.ts      |  6 ++--
 .../sagas-and-records/src/models/Pet.ts       | 30 +++++++++----------
 .../src/models/PetRegionsResponse.ts          |  4 +--
 .../src/models/ResponseMeta.ts                |  4 +--
 .../sagas-and-records/src/models/Tag.ts       |  2 +-
 .../sagas-and-records/src/models/User.ts      |  6 ++--
 .../models/AdditionalPropertiesClass.ts       |  2 +-
 .../models/AllOfWithSingleRef.ts              |  2 +-
 .../snakecase-discriminator/models/Animal.ts  |  4 +--
 .../models/ArrayOfArrayOfNumberOnly.ts        |  2 +-
 .../models/ArrayOfNumberOnly.ts               |  2 +-
 .../models/ArrayTest.ts                       |  2 +-
 .../models/Capitalization.ts                  |  2 +-
 .../snakecase-discriminator/models/Cat.ts     |  2 +-
 .../models/Category.ts                        |  4 +--
 .../models/ClassModel.ts                      |  2 +-
 .../snakecase-discriminator/models/Client.ts  |  2 +-
 .../models/DeprecatedObject.ts                |  2 +-
 .../snakecase-discriminator/models/Dog.ts     |  2 +-
 .../models/EnumArrays.ts                      |  2 +-
 .../models/EnumTest.ts                        |  4 +--
 .../models/FakeBigDecimalMap200Response.ts    |  2 +-
 .../models/FileSchemaTestClass.ts             |  2 +-
 .../snakecase-discriminator/models/Foo.ts     |  2 +-
 .../models/FooGetDefaultResponse.ts           |  2 +-
 .../models/FormatTest.ts                      | 10 +++----
 .../models/HasOnlyReadOnly.ts                 |  2 +-
 .../models/HealthCheckResult.ts               |  2 +-
 .../snakecase-discriminator/models/List.ts    |  2 +-
 .../snakecase-discriminator/models/MapTest.ts |  2 +-
 ...dPropertiesAndAdditionalPropertiesClass.ts |  2 +-
 .../models/Model200Response.ts                |  2 +-
 .../models/ModelApiResponse.ts                |  2 +-
 .../models/ModelFile.ts                       |  2 +-
 .../snakecase-discriminator/models/Name.ts    |  4 +--
 .../models/NullableClass.ts                   |  2 +-
 .../models/NumberOnly.ts                      |  2 +-
 .../models/ObjectWithDeprecatedFields.ts      |  2 +-
 .../snakecase-discriminator/models/Order.ts   |  2 +-
 .../models/OuterComposite.ts                  |  2 +-
 .../models/OuterObjectWithEnumProperty.ts     |  4 +--
 .../snakecase-discriminator/models/Pet.ts     |  6 ++--
 .../models/ReadOnlyFirst.ts                   |  2 +-
 .../snakecase-discriminator/models/Return.ts  |  2 +-
 .../models/SpecialModelName.ts                |  2 +-
 .../snakecase-discriminator/models/Tag.ts     |  2 +-
 .../snakecase-discriminator/models/User.ts    |  2 +-
 .../builds/with-interfaces/models/Category.ts |  2 +-
 .../models/ModelApiResponse.ts                |  2 +-
 .../builds/with-interfaces/models/Order.ts    |  2 +-
 .../builds/with-interfaces/models/Pet.ts      |  6 ++--
 .../builds/with-interfaces/models/Tag.ts      |  2 +-
 .../builds/with-interfaces/models/User.ts     |  2 +-
 .../with-npm-version/src/models/Category.ts   |  2 +-
 .../src/models/ModelApiResponse.ts            |  2 +-
 .../with-npm-version/src/models/Order.ts      |  2 +-
 .../builds/with-npm-version/src/models/Pet.ts |  6 ++--
 .../builds/with-npm-version/src/models/Tag.ts |  2 +-
 .../with-npm-version/src/models/User.ts       |  2 +-
 .../models/EnumPatternObject.ts               |  2 +-
 .../FakeEnumRequestGetInline200Response.ts    |  2 +-
 149 files changed, 216 insertions(+), 216 deletions(-)

diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
index 8e7d0344cd4a..0228aa5374e1 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
@@ -20,10 +20,10 @@ import { {{modelName}}FromJSONTyped } from './{{modelName}}{{importFileExtension
 /**
  * Check if a given object implements the {{classname}} interface.
  */
-export function instanceOf{{classname}}(value: object): boolean {
+export function instanceOf{{classname}}(value: object): value is {{classname}} {
     {{#vars}}
     {{#required}}
-    if (!('{{name}}' in value)) return false;
+    if (!('{{name}}' in value) || value['{{name}}'] === undefined) return false;
     {{/required}}
     {{/vars}}
     return true;
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
index 5e7c7f7f972f..6f935b2048d8 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
@@ -37,7 +37,7 @@ export interface Club {
 /**
  * Check if a given object implements the Club interface.
  */
-export function instanceOfClub(value: object): boolean {
+export function instanceOfClub(value: object): value is Club {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
index 50e770a7d3f8..58df8cbac491 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
@@ -30,7 +30,7 @@ export interface Owner {
 /**
  * Check if a given object implements the Owner interface.
  */
-export function instanceOfOwner(value: object): boolean {
+export function instanceOfOwner(value: object): value is Owner {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
index d61b559fea2a..c35b4f285e3f 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
@@ -37,7 +37,7 @@ export interface Club {
 /**
  * Check if a given object implements the Club interface.
  */
-export function instanceOfClub(value: object): boolean {
+export function instanceOfClub(value: object): value is Club {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
index 50e770a7d3f8..58df8cbac491 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
@@ -30,7 +30,7 @@ export interface Owner {
 /**
  * Check if a given object implements the Owner interface.
  */
-export function instanceOfOwner(value: object): boolean {
+export function instanceOfOwner(value: object): value is Owner {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
index f0c934a942c6..a83efbc1155a 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
@@ -36,7 +36,7 @@ export interface AdditionalPropertiesClass {
 /**
  * Check if a given object implements the AdditionalPropertiesClass interface.
  */
-export function instanceOfAdditionalPropertiesClass(value: object): boolean {
+export function instanceOfAdditionalPropertiesClass(value: object): value is AdditionalPropertiesClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
index 8f9802529e4b..2b247b983ffb 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
@@ -43,7 +43,7 @@ export interface AllOfWithSingleRef {
 /**
  * Check if a given object implements the AllOfWithSingleRef interface.
  */
-export function instanceOfAllOfWithSingleRef(value: object): boolean {
+export function instanceOfAllOfWithSingleRef(value: object): value is AllOfWithSingleRef {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
index a5f85d1cc610..236ba544d0d8 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
@@ -38,8 +38,8 @@ export interface Animal {
 /**
  * Check if a given object implements the Animal interface.
  */
-export function instanceOfAnimal(value: object): boolean {
-    if (!('className' in value)) return false;
+export function instanceOfAnimal(value: object): value is Animal {
+    if (!('className' in value) || value['className'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
index 6e4c4ac139c3..dbffd74a5723 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
@@ -30,7 +30,7 @@ export interface ArrayOfArrayOfNumberOnly {
 /**
  * Check if a given object implements the ArrayOfArrayOfNumberOnly interface.
  */
-export function instanceOfArrayOfArrayOfNumberOnly(value: object): boolean {
+export function instanceOfArrayOfArrayOfNumberOnly(value: object): value is ArrayOfArrayOfNumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
index 4c0e2e98805b..8029e9dd15bc 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
@@ -30,7 +30,7 @@ export interface ArrayOfNumberOnly {
 /**
  * Check if a given object implements the ArrayOfNumberOnly interface.
  */
-export function instanceOfArrayOfNumberOnly(value: object): boolean {
+export function instanceOfArrayOfNumberOnly(value: object): value is ArrayOfNumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
index 2cdb8f33fa6f..fefc0e842b6f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
@@ -49,7 +49,7 @@ export interface ArrayTest {
 /**
  * Check if a given object implements the ArrayTest interface.
  */
-export function instanceOfArrayTest(value: object): boolean {
+export function instanceOfArrayTest(value: object): value is ArrayTest {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
index 1d7e59d6099d..f9f38b2ed71f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
@@ -61,7 +61,7 @@ export interface Capitalization {
 /**
  * Check if a given object implements the Capitalization interface.
  */
-export function instanceOfCapitalization(value: object): boolean {
+export function instanceOfCapitalization(value: object): value is Capitalization {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
index 3bc2bd8cdb13..05ffd9e8fab5 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
@@ -37,7 +37,7 @@ export interface Cat extends Animal {
 /**
  * Check if a given object implements the Cat interface.
  */
-export function instanceOfCat(value: object): boolean {
+export function instanceOfCat(value: object): value is Cat {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
index e571880c8e1e..640b1883c524 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
@@ -36,8 +36,8 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
-    if (!('name' in value)) return false;
+export function instanceOfCategory(value: object): value is Category {
+    if (!('name' in value) || value['name'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
index 2ed247a45410..24167b77a66b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
@@ -39,7 +39,7 @@ export interface ChildWithNullable extends ParentWithNullable {
 /**
  * Check if a given object implements the ChildWithNullable interface.
  */
-export function instanceOfChildWithNullable(value: object): boolean {
+export function instanceOfChildWithNullable(value: object): value is ChildWithNullable {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
index 6d9ad88c1c55..1789763aaecc 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
@@ -30,7 +30,7 @@ export interface ClassModel {
 /**
  * Check if a given object implements the ClassModel interface.
  */
-export function instanceOfClassModel(value: object): boolean {
+export function instanceOfClassModel(value: object): value is ClassModel {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
index 807cd0777308..21c3a1f3b798 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
@@ -30,7 +30,7 @@ export interface Client {
 /**
  * Check if a given object implements the Client interface.
  */
-export function instanceOfClient(value: object): boolean {
+export function instanceOfClient(value: object): value is Client {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
index 8eab37673066..38ae523f5b96 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
@@ -30,7 +30,7 @@ export interface DeprecatedObject {
 /**
  * Check if a given object implements the DeprecatedObject interface.
  */
-export function instanceOfDeprecatedObject(value: object): boolean {
+export function instanceOfDeprecatedObject(value: object): value is DeprecatedObject {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
index 921e17cb803f..5df4d5123a82 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
@@ -37,7 +37,7 @@ export interface Dog extends Animal {
 /**
  * Check if a given object implements the Dog interface.
  */
-export function instanceOfDog(value: object): boolean {
+export function instanceOfDog(value: object): value is Dog {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
index aa7200c92542..ddff5eeb796e 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
@@ -56,7 +56,7 @@ export type EnumArraysArrayEnumEnum = typeof EnumArraysArrayEnumEnum[keyof typeo
 /**
  * Check if a given object implements the EnumArrays interface.
  */
-export function instanceOfEnumArrays(value: object): boolean {
+export function instanceOfEnumArrays(value: object): value is EnumArrays {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
index caeccfc55388..b4ef265fd2d2 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
@@ -137,8 +137,8 @@ export type EnumTestEnumNumberEnum = typeof EnumTestEnumNumberEnum[keyof typeof
 /**
  * Check if a given object implements the EnumTest interface.
  */
-export function instanceOfEnumTest(value: object): boolean {
-    if (!('enumStringRequired' in value)) return false;
+export function instanceOfEnumTest(value: object): value is EnumTest {
+    if (!('enumStringRequired' in value) || value['enumStringRequired'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
index 732ed05b62a0..dc65ebf50f77 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
@@ -36,7 +36,7 @@ export interface FakeBigDecimalMap200Response {
 /**
  * Check if a given object implements the FakeBigDecimalMap200Response interface.
  */
-export function instanceOfFakeBigDecimalMap200Response(value: object): boolean {
+export function instanceOfFakeBigDecimalMap200Response(value: object): value is FakeBigDecimalMap200Response {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
index 25bf3ba4dbf1..9da5b93a1a20 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
@@ -36,7 +36,7 @@ export interface FileSchemaTestClass {
 /**
  * Check if a given object implements the FileSchemaTestClass interface.
  */
-export function instanceOfFileSchemaTestClass(value: object): boolean {
+export function instanceOfFileSchemaTestClass(value: object): value is FileSchemaTestClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
index feb5e47da049..94845a52698d 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
@@ -30,7 +30,7 @@ export interface Foo {
 /**
  * Check if a given object implements the Foo interface.
  */
-export function instanceOfFoo(value: object): boolean {
+export function instanceOfFoo(value: object): value is Foo {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
index b834ab550db6..86af8d7e7cc0 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
@@ -37,7 +37,7 @@ export interface FooGetDefaultResponse {
 /**
  * Check if a given object implements the FooGetDefaultResponse interface.
  */
-export function instanceOfFooGetDefaultResponse(value: object): boolean {
+export function instanceOfFooGetDefaultResponse(value: object): value is FooGetDefaultResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
index f2113709741f..328ec809ee95 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
@@ -127,11 +127,11 @@ export interface FormatTest {
 /**
  * Check if a given object implements the FormatTest interface.
  */
-export function instanceOfFormatTest(value: object): boolean {
-    if (!('number' in value)) return false;
-    if (!('_byte' in value)) return false;
-    if (!('date' in value)) return false;
-    if (!('password' in value)) return false;
+export function instanceOfFormatTest(value: object): value is FormatTest {
+    if (!('number' in value) || value['number'] === undefined) return false;
+    if (!('_byte' in value) || value['_byte'] === undefined) return false;
+    if (!('date' in value) || value['date'] === undefined) return false;
+    if (!('password' in value) || value['password'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
index 82e1d7235e13..db19db8f72cf 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
@@ -36,7 +36,7 @@ export interface HasOnlyReadOnly {
 /**
  * Check if a given object implements the HasOnlyReadOnly interface.
  */
-export function instanceOfHasOnlyReadOnly(value: object): boolean {
+export function instanceOfHasOnlyReadOnly(value: object): value is HasOnlyReadOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
index 83a0cf52ee15..450b2c25043b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
@@ -30,7 +30,7 @@ export interface HealthCheckResult {
 /**
  * Check if a given object implements the HealthCheckResult interface.
  */
-export function instanceOfHealthCheckResult(value: object): boolean {
+export function instanceOfHealthCheckResult(value: object): value is HealthCheckResult {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
index 5513deda0790..2b979862b769 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
@@ -30,7 +30,7 @@ export interface List {
 /**
  * Check if a given object implements the List interface.
  */
-export function instanceOfList(value: object): boolean {
+export function instanceOfList(value: object): value is List {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
index 0ba03f9a901b..7a2a4b27fef8 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
@@ -59,7 +59,7 @@ export type MapTestMapOfEnumStringEnum = typeof MapTestMapOfEnumStringEnum[keyof
 /**
  * Check if a given object implements the MapTest interface.
  */
-export function instanceOfMapTest(value: object): boolean {
+export function instanceOfMapTest(value: object): value is MapTest {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index 156ccc8a342d..599422a3417b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
@@ -49,7 +49,7 @@ export interface MixedPropertiesAndAdditionalPropertiesClass {
 /**
  * Check if a given object implements the MixedPropertiesAndAdditionalPropertiesClass interface.
  */
-export function instanceOfMixedPropertiesAndAdditionalPropertiesClass(value: object): boolean {
+export function instanceOfMixedPropertiesAndAdditionalPropertiesClass(value: object): value is MixedPropertiesAndAdditionalPropertiesClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
index 22570495bd69..31254e14b766 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
@@ -36,7 +36,7 @@ export interface Model200Response {
 /**
  * Check if a given object implements the Model200Response interface.
  */
-export function instanceOfModel200Response(value: object): boolean {
+export function instanceOfModel200Response(value: object): value is Model200Response {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
index 7116a59f8d22..bf92ceff548f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
index dc08a2f1474a..c54577e4fc06 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
@@ -30,7 +30,7 @@ export interface ModelFile {
 /**
  * Check if a given object implements the ModelFile interface.
  */
-export function instanceOfModelFile(value: object): boolean {
+export function instanceOfModelFile(value: object): value is ModelFile {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
index 9b861d6d4d14..dbcf899e8541 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
@@ -48,8 +48,8 @@ export interface Name {
 /**
  * Check if a given object implements the Name interface.
  */
-export function instanceOfName(value: object): boolean {
-    if (!('name' in value)) return false;
+export function instanceOfName(value: object): value is Name {
+    if (!('name' in value) || value['name'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
index b64cd7d8f452..2fbcea7c6eac 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
@@ -97,7 +97,7 @@ export interface NullableClass {
 /**
  * Check if a given object implements the NullableClass interface.
  */
-export function instanceOfNullableClass(value: object): boolean {
+export function instanceOfNullableClass(value: object): value is NullableClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
index 155afb118e43..c778f996c1a2 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
@@ -30,7 +30,7 @@ export interface NumberOnly {
 /**
  * Check if a given object implements the NumberOnly interface.
  */
-export function instanceOfNumberOnly(value: object): boolean {
+export function instanceOfNumberOnly(value: object): value is NumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
index d24ea8fc197e..99c4b488c489 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
@@ -58,7 +58,7 @@ export interface ObjectWithDeprecatedFields {
 /**
  * Check if a given object implements the ObjectWithDeprecatedFields interface.
  */
-export function instanceOfObjectWithDeprecatedFields(value: object): boolean {
+export function instanceOfObjectWithDeprecatedFields(value: object): value is ObjectWithDeprecatedFields {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
index 8921894971d6..7b83e804baad 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
index 49d8ad5cc5b1..c965a360d04b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
@@ -42,7 +42,7 @@ export interface OuterComposite {
 /**
  * Check if a given object implements the OuterComposite interface.
  */
-export function instanceOfOuterComposite(value: object): boolean {
+export function instanceOfOuterComposite(value: object): value is OuterComposite {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
index 3990064c3484..c8ca1734039f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
@@ -37,8 +37,8 @@ export interface OuterObjectWithEnumProperty {
 /**
  * Check if a given object implements the OuterObjectWithEnumProperty interface.
  */
-export function instanceOfOuterObjectWithEnumProperty(value: object): boolean {
-    if (!('value' in value)) return false;
+export function instanceOfOuterObjectWithEnumProperty(value: object): value is OuterObjectWithEnumProperty {
+    if (!('value' in value) || value['value'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
index c549674ec4a2..81195930614e 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
@@ -47,7 +47,7 @@ export type ParentWithNullableTypeEnum = typeof ParentWithNullableTypeEnum[keyof
 /**
  * Check if a given object implements the ParentWithNullable interface.
  */
-export function instanceOfParentWithNullable(value: object): boolean {
+export function instanceOfParentWithNullable(value: object): value is ParentWithNullable {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
index 1f29621cb3b4..f2ad3c1ee466 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
index f1f5a01a89b6..9bd46c432fca 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
@@ -36,7 +36,7 @@ export interface ReadOnlyFirst {
 /**
  * Check if a given object implements the ReadOnlyFirst interface.
  */
-export function instanceOfReadOnlyFirst(value: object): boolean {
+export function instanceOfReadOnlyFirst(value: object): value is ReadOnlyFirst {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
index 68b15542cbbd..3de8fb2d62be 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
@@ -30,7 +30,7 @@ export interface Return {
 /**
  * Check if a given object implements the Return interface.
  */
-export function instanceOfReturn(value: object): boolean {
+export function instanceOfReturn(value: object): value is Return {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
index b302e6f03627..34edcaa2f004 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
@@ -30,7 +30,7 @@ export interface SpecialModelName {
 /**
  * Check if a given object implements the SpecialModelName interface.
  */
-export function instanceOfSpecialModelName(value: object): boolean {
+export function instanceOfSpecialModelName(value: object): value is SpecialModelName {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
index 4ae4af399fd8..f0ecd59d5ba2 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
index acc16c94f495..cfe6fac4f43f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
@@ -31,7 +31,7 @@ export interface TestInlineFreeformAdditionalPropertiesRequest {
 /**
  * Check if a given object implements the TestInlineFreeformAdditionalPropertiesRequest interface.
  */
-export function instanceOfTestInlineFreeformAdditionalPropertiesRequest(value: object): boolean {
+export function instanceOfTestInlineFreeformAdditionalPropertiesRequest(value: object): value is TestInlineFreeformAdditionalPropertiesRequest {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
index c85ea3d6ed63..2028044d626c 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/User.ts b/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts b/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
index a902fb9c9af3..8f61f66faaa1 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
@@ -61,7 +61,7 @@ export interface EnumPatternObject {
 /**
  * Check if a given object implements the EnumPatternObject interface.
  */
-export function instanceOfEnumPatternObject(value: object): boolean {
+export function instanceOfEnumPatternObject(value: object): value is EnumPatternObject {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts b/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
index 0e53bcfd09d6..644637105ce5 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
@@ -90,7 +90,7 @@ export type FakeEnumRequestGetInline200ResponseNullableNumberEnumEnum = typeof F
 /**
  * Check if a given object implements the FakeEnumRequestGetInline200Response interface.
  */
-export function instanceOfFakeEnumRequestGetInline200Response(value: object): boolean {
+export function instanceOfFakeEnumRequestGetInline200Response(value: object): value is FakeEnumRequestGetInline200Response {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
index 72d0569f4ccd..9e3a2d63c543 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
@@ -37,8 +37,8 @@ export interface DefaultMetaOnlyResponse {
 /**
  * Check if a given object implements the DefaultMetaOnlyResponse interface.
  */
-export function instanceOfDefaultMetaOnlyResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfDefaultMetaOnlyResponse(value: object): value is DefaultMetaOnlyResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
index 8101bbc1d26e..7f4307dbd771 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
@@ -49,8 +49,8 @@ export interface FindPetsByStatusResponse {
 /**
  * Check if a given object implements the FindPetsByStatusResponse interface.
  */
-export function instanceOfFindPetsByStatusResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfFindPetsByStatusResponse(value: object): value is FindPetsByStatusResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
index 83b0d3308f0f..31e86f68d609 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
@@ -49,8 +49,8 @@ export interface FindPetsByUserResponse {
 /**
  * Check if a given object implements the FindPetsByUserResponse interface.
  */
-export function instanceOfFindPetsByUserResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfFindPetsByUserResponse(value: object): value is FindPetsByUserResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
index e25682be1f59..c001a843b0e5 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
@@ -43,8 +43,8 @@ export interface GetBehaviorPermissionsResponse {
 /**
  * Check if a given object implements the GetBehaviorPermissionsResponse interface.
  */
-export function instanceOfGetBehaviorPermissionsResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfGetBehaviorPermissionsResponse(value: object): value is GetBehaviorPermissionsResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
index 4f00ef278937..5de27f58461d 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
@@ -49,8 +49,8 @@ export interface GetBehaviorTypeResponse {
 /**
  * Check if a given object implements the GetBehaviorTypeResponse interface.
  */
-export function instanceOfGetBehaviorTypeResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfGetBehaviorTypeResponse(value: object): value is GetBehaviorTypeResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
index aef34f1254fc..af60c4b351a0 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
@@ -49,8 +49,8 @@ export interface GetMatchingPartsResponse {
 /**
  * Check if a given object implements the GetMatchingPartsResponse interface.
  */
-export function instanceOfGetMatchingPartsResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfGetMatchingPartsResponse(value: object): value is GetMatchingPartsResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
index 2b35d9f88880..16ec39476732 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
@@ -49,8 +49,8 @@ export interface GetPetPartTypeResponse {
 /**
  * Check if a given object implements the GetPetPartTypeResponse interface.
  */
-export function instanceOfGetPetPartTypeResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfGetPetPartTypeResponse(value: object): value is GetPetPartTypeResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
index 5c2c26352428..bad63a3c68c5 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
@@ -36,9 +36,9 @@ export interface ItemId {
 /**
  * Check if a given object implements the ItemId interface.
  */
-export function instanceOfItemId(value: object): boolean {
-    if (!('id' in value)) return false;
-    if (!('type' in value)) return false;
+export function instanceOfItemId(value: object): value is ItemId {
+    if (!('id' in value) || value['id'] === undefined) return false;
+    if (!('type' in value) || value['type'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
index 6c27dd2e8019..7f501316a407 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
@@ -43,9 +43,9 @@ export interface MatchingParts {
 /**
  * Check if a given object implements the MatchingParts interface.
  */
-export function instanceOfMatchingParts(value: object): boolean {
-    if (!('connected' in value)) return false;
-    if (!('related' in value)) return false;
+export function instanceOfMatchingParts(value: object): value is MatchingParts {
+    if (!('connected' in value) || value['connected'] === undefined) return false;
+    if (!('related' in value) || value['related'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
index 75714ef92cce..d5b17c2160c2 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
@@ -55,8 +55,8 @@ export interface ModelError {
 /**
  * Check if a given object implements the ModelError interface.
  */
-export function instanceOfModelError(value: object): boolean {
-    if (!('type' in value)) return false;
+export function instanceOfModelError(value: object): value is ModelError {
+    if (!('type' in value) || value['type'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
index 9d7ae4222138..07f345d28df0 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
@@ -36,9 +36,9 @@ export interface Part {
 /**
  * Check if a given object implements the Part interface.
  */
-export function instanceOfPart(value: object): boolean {
-    if (!('id' in value)) return false;
-    if (!('name' in value)) return false;
+export function instanceOfPart(value: object): value is Part {
+    if (!('id' in value) || value['id'] === undefined) return false;
+    if (!('name' in value) || value['name'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
index bc567b494366..35c173b55b36 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
@@ -187,21 +187,21 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('id' in value)) return false;
-    if (!('otherFriendIds' in value)) return false;
-    if (!('friendAge' in value)) return false;
-    if (!('age' in value)) return false;
-    if (!('isHappy' in value)) return false;
-    if (!('isTall' in value)) return false;
-    if (!('category' in value)) return false;
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
-    if (!('warningStatus' in value)) return false;
-    if (!('alternateStatus' in value)) return false;
-    if (!('otherDepStatuses' in value)) return false;
-    if (!('tags' in value)) return false;
-    if (!('status' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('id' in value) || value['id'] === undefined) return false;
+    if (!('otherFriendIds' in value) || value['otherFriendIds'] === undefined) return false;
+    if (!('friendAge' in value) || value['friendAge'] === undefined) return false;
+    if (!('age' in value) || value['age'] === undefined) return false;
+    if (!('isHappy' in value) || value['isHappy'] === undefined) return false;
+    if (!('isTall' in value) || value['isTall'] === undefined) return false;
+    if (!('category' in value) || value['category'] === undefined) return false;
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
+    if (!('warningStatus' in value) || value['warningStatus'] === undefined) return false;
+    if (!('alternateStatus' in value) || value['alternateStatus'] === undefined) return false;
+    if (!('otherDepStatuses' in value) || value['otherDepStatuses'] === undefined) return false;
+    if (!('tags' in value) || value['tags'] === undefined) return false;
+    if (!('status' in value) || value['status'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
index 400a5fa83e8d..8430854a46ce 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
@@ -43,8 +43,8 @@ export interface PetRegionsResponse {
 /**
  * Check if a given object implements the PetRegionsResponse interface.
  */
-export function instanceOfPetRegionsResponse(value: object): boolean {
-    if (!('meta' in value)) return false;
+export function instanceOfPetRegionsResponse(value: object): value is PetRegionsResponse {
+    if (!('meta' in value) || value['meta'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
index 6b17720e996a..4584d592e7cc 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
@@ -99,8 +99,8 @@ export type ResponseMetaCodeEnum = typeof ResponseMetaCodeEnum[keyof typeof Resp
 /**
  * Check if a given object implements the ResponseMeta interface.
  */
-export function instanceOfResponseMeta(value: object): boolean {
-    if (!('code' in value)) return false;
+export function instanceOfResponseMeta(value: object): value is ResponseMeta {
+    if (!('code' in value) || value['code'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
index c6cb145be14e..17092c2c5804 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
@@ -84,9 +84,9 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
-    if (!('id' in value)) return false;
-    if (!('subUser2' in value)) return false;
+export function instanceOfUser(value: object): value is User {
+    if (!('id' in value) || value['id'] === undefined) return false;
+    if (!('subUser2' in value) || value['subUser2'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
index f0c934a942c6..a83efbc1155a 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
@@ -36,7 +36,7 @@ export interface AdditionalPropertiesClass {
 /**
  * Check if a given object implements the AdditionalPropertiesClass interface.
  */
-export function instanceOfAdditionalPropertiesClass(value: object): boolean {
+export function instanceOfAdditionalPropertiesClass(value: object): value is AdditionalPropertiesClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
index 8f9802529e4b..2b247b983ffb 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
@@ -43,7 +43,7 @@ export interface AllOfWithSingleRef {
 /**
  * Check if a given object implements the AllOfWithSingleRef interface.
  */
-export function instanceOfAllOfWithSingleRef(value: object): boolean {
+export function instanceOfAllOfWithSingleRef(value: object): value is AllOfWithSingleRef {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
index 5edd6e54feea..a49466dfac66 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
@@ -38,8 +38,8 @@ export interface Animal {
 /**
  * Check if a given object implements the Animal interface.
  */
-export function instanceOfAnimal(value: object): boolean {
-    if (!('className' in value)) return false;
+export function instanceOfAnimal(value: object): value is Animal {
+    if (!('className' in value) || value['className'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
index 6e4c4ac139c3..dbffd74a5723 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
@@ -30,7 +30,7 @@ export interface ArrayOfArrayOfNumberOnly {
 /**
  * Check if a given object implements the ArrayOfArrayOfNumberOnly interface.
  */
-export function instanceOfArrayOfArrayOfNumberOnly(value: object): boolean {
+export function instanceOfArrayOfArrayOfNumberOnly(value: object): value is ArrayOfArrayOfNumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
index 4c0e2e98805b..8029e9dd15bc 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
@@ -30,7 +30,7 @@ export interface ArrayOfNumberOnly {
 /**
  * Check if a given object implements the ArrayOfNumberOnly interface.
  */
-export function instanceOfArrayOfNumberOnly(value: object): boolean {
+export function instanceOfArrayOfNumberOnly(value: object): value is ArrayOfNumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
index 2cdb8f33fa6f..fefc0e842b6f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
@@ -49,7 +49,7 @@ export interface ArrayTest {
 /**
  * Check if a given object implements the ArrayTest interface.
  */
-export function instanceOfArrayTest(value: object): boolean {
+export function instanceOfArrayTest(value: object): value is ArrayTest {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
index 1d7e59d6099d..f9f38b2ed71f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
@@ -61,7 +61,7 @@ export interface Capitalization {
 /**
  * Check if a given object implements the Capitalization interface.
  */
-export function instanceOfCapitalization(value: object): boolean {
+export function instanceOfCapitalization(value: object): value is Capitalization {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
index 3bc2bd8cdb13..05ffd9e8fab5 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
@@ -37,7 +37,7 @@ export interface Cat extends Animal {
 /**
  * Check if a given object implements the Cat interface.
  */
-export function instanceOfCat(value: object): boolean {
+export function instanceOfCat(value: object): value is Cat {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
index e571880c8e1e..640b1883c524 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
@@ -36,8 +36,8 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
-    if (!('name' in value)) return false;
+export function instanceOfCategory(value: object): value is Category {
+    if (!('name' in value) || value['name'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
index 6d9ad88c1c55..1789763aaecc 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
@@ -30,7 +30,7 @@ export interface ClassModel {
 /**
  * Check if a given object implements the ClassModel interface.
  */
-export function instanceOfClassModel(value: object): boolean {
+export function instanceOfClassModel(value: object): value is ClassModel {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
index 807cd0777308..21c3a1f3b798 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
@@ -30,7 +30,7 @@ export interface Client {
 /**
  * Check if a given object implements the Client interface.
  */
-export function instanceOfClient(value: object): boolean {
+export function instanceOfClient(value: object): value is Client {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
index 8eab37673066..38ae523f5b96 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
@@ -30,7 +30,7 @@ export interface DeprecatedObject {
 /**
  * Check if a given object implements the DeprecatedObject interface.
  */
-export function instanceOfDeprecatedObject(value: object): boolean {
+export function instanceOfDeprecatedObject(value: object): value is DeprecatedObject {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
index 921e17cb803f..5df4d5123a82 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
@@ -37,7 +37,7 @@ export interface Dog extends Animal {
 /**
  * Check if a given object implements the Dog interface.
  */
-export function instanceOfDog(value: object): boolean {
+export function instanceOfDog(value: object): value is Dog {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
index aa7200c92542..ddff5eeb796e 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
@@ -56,7 +56,7 @@ export type EnumArraysArrayEnumEnum = typeof EnumArraysArrayEnumEnum[keyof typeo
 /**
  * Check if a given object implements the EnumArrays interface.
  */
-export function instanceOfEnumArrays(value: object): boolean {
+export function instanceOfEnumArrays(value: object): value is EnumArrays {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
index caeccfc55388..b4ef265fd2d2 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
@@ -137,8 +137,8 @@ export type EnumTestEnumNumberEnum = typeof EnumTestEnumNumberEnum[keyof typeof
 /**
  * Check if a given object implements the EnumTest interface.
  */
-export function instanceOfEnumTest(value: object): boolean {
-    if (!('enumStringRequired' in value)) return false;
+export function instanceOfEnumTest(value: object): value is EnumTest {
+    if (!('enumStringRequired' in value) || value['enumStringRequired'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
index 732ed05b62a0..dc65ebf50f77 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
@@ -36,7 +36,7 @@ export interface FakeBigDecimalMap200Response {
 /**
  * Check if a given object implements the FakeBigDecimalMap200Response interface.
  */
-export function instanceOfFakeBigDecimalMap200Response(value: object): boolean {
+export function instanceOfFakeBigDecimalMap200Response(value: object): value is FakeBigDecimalMap200Response {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
index 25bf3ba4dbf1..9da5b93a1a20 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
@@ -36,7 +36,7 @@ export interface FileSchemaTestClass {
 /**
  * Check if a given object implements the FileSchemaTestClass interface.
  */
-export function instanceOfFileSchemaTestClass(value: object): boolean {
+export function instanceOfFileSchemaTestClass(value: object): value is FileSchemaTestClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
index feb5e47da049..94845a52698d 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
@@ -30,7 +30,7 @@ export interface Foo {
 /**
  * Check if a given object implements the Foo interface.
  */
-export function instanceOfFoo(value: object): boolean {
+export function instanceOfFoo(value: object): value is Foo {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
index b834ab550db6..86af8d7e7cc0 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
@@ -37,7 +37,7 @@ export interface FooGetDefaultResponse {
 /**
  * Check if a given object implements the FooGetDefaultResponse interface.
  */
-export function instanceOfFooGetDefaultResponse(value: object): boolean {
+export function instanceOfFooGetDefaultResponse(value: object): value is FooGetDefaultResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
index f2113709741f..328ec809ee95 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
@@ -127,11 +127,11 @@ export interface FormatTest {
 /**
  * Check if a given object implements the FormatTest interface.
  */
-export function instanceOfFormatTest(value: object): boolean {
-    if (!('number' in value)) return false;
-    if (!('_byte' in value)) return false;
-    if (!('date' in value)) return false;
-    if (!('password' in value)) return false;
+export function instanceOfFormatTest(value: object): value is FormatTest {
+    if (!('number' in value) || value['number'] === undefined) return false;
+    if (!('_byte' in value) || value['_byte'] === undefined) return false;
+    if (!('date' in value) || value['date'] === undefined) return false;
+    if (!('password' in value) || value['password'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
index 82e1d7235e13..db19db8f72cf 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
@@ -36,7 +36,7 @@ export interface HasOnlyReadOnly {
 /**
  * Check if a given object implements the HasOnlyReadOnly interface.
  */
-export function instanceOfHasOnlyReadOnly(value: object): boolean {
+export function instanceOfHasOnlyReadOnly(value: object): value is HasOnlyReadOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
index 83a0cf52ee15..450b2c25043b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
@@ -30,7 +30,7 @@ export interface HealthCheckResult {
 /**
  * Check if a given object implements the HealthCheckResult interface.
  */
-export function instanceOfHealthCheckResult(value: object): boolean {
+export function instanceOfHealthCheckResult(value: object): value is HealthCheckResult {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
index 5513deda0790..2b979862b769 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
@@ -30,7 +30,7 @@ export interface List {
 /**
  * Check if a given object implements the List interface.
  */
-export function instanceOfList(value: object): boolean {
+export function instanceOfList(value: object): value is List {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
index 0ba03f9a901b..7a2a4b27fef8 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
@@ -59,7 +59,7 @@ export type MapTestMapOfEnumStringEnum = typeof MapTestMapOfEnumStringEnum[keyof
 /**
  * Check if a given object implements the MapTest interface.
  */
-export function instanceOfMapTest(value: object): boolean {
+export function instanceOfMapTest(value: object): value is MapTest {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index 156ccc8a342d..599422a3417b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
@@ -49,7 +49,7 @@ export interface MixedPropertiesAndAdditionalPropertiesClass {
 /**
  * Check if a given object implements the MixedPropertiesAndAdditionalPropertiesClass interface.
  */
-export function instanceOfMixedPropertiesAndAdditionalPropertiesClass(value: object): boolean {
+export function instanceOfMixedPropertiesAndAdditionalPropertiesClass(value: object): value is MixedPropertiesAndAdditionalPropertiesClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
index 22570495bd69..31254e14b766 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
@@ -36,7 +36,7 @@ export interface Model200Response {
 /**
  * Check if a given object implements the Model200Response interface.
  */
-export function instanceOfModel200Response(value: object): boolean {
+export function instanceOfModel200Response(value: object): value is Model200Response {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
index 7116a59f8d22..bf92ceff548f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
index dc08a2f1474a..c54577e4fc06 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
@@ -30,7 +30,7 @@ export interface ModelFile {
 /**
  * Check if a given object implements the ModelFile interface.
  */
-export function instanceOfModelFile(value: object): boolean {
+export function instanceOfModelFile(value: object): value is ModelFile {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
index 9b861d6d4d14..dbcf899e8541 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
@@ -48,8 +48,8 @@ export interface Name {
 /**
  * Check if a given object implements the Name interface.
  */
-export function instanceOfName(value: object): boolean {
-    if (!('name' in value)) return false;
+export function instanceOfName(value: object): value is Name {
+    if (!('name' in value) || value['name'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
index b64cd7d8f452..2fbcea7c6eac 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
@@ -97,7 +97,7 @@ export interface NullableClass {
 /**
  * Check if a given object implements the NullableClass interface.
  */
-export function instanceOfNullableClass(value: object): boolean {
+export function instanceOfNullableClass(value: object): value is NullableClass {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
index 155afb118e43..c778f996c1a2 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
@@ -30,7 +30,7 @@ export interface NumberOnly {
 /**
  * Check if a given object implements the NumberOnly interface.
  */
-export function instanceOfNumberOnly(value: object): boolean {
+export function instanceOfNumberOnly(value: object): value is NumberOnly {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
index d24ea8fc197e..99c4b488c489 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
@@ -58,7 +58,7 @@ export interface ObjectWithDeprecatedFields {
 /**
  * Check if a given object implements the ObjectWithDeprecatedFields interface.
  */
-export function instanceOfObjectWithDeprecatedFields(value: object): boolean {
+export function instanceOfObjectWithDeprecatedFields(value: object): value is ObjectWithDeprecatedFields {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
index 8921894971d6..7b83e804baad 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
index 49d8ad5cc5b1..c965a360d04b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
@@ -42,7 +42,7 @@ export interface OuterComposite {
 /**
  * Check if a given object implements the OuterComposite interface.
  */
-export function instanceOfOuterComposite(value: object): boolean {
+export function instanceOfOuterComposite(value: object): value is OuterComposite {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
index 3990064c3484..c8ca1734039f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
@@ -37,8 +37,8 @@ export interface OuterObjectWithEnumProperty {
 /**
  * Check if a given object implements the OuterObjectWithEnumProperty interface.
  */
-export function instanceOfOuterObjectWithEnumProperty(value: object): boolean {
-    if (!('value' in value)) return false;
+export function instanceOfOuterObjectWithEnumProperty(value: object): value is OuterObjectWithEnumProperty {
+    if (!('value' in value) || value['value'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
index 1f29621cb3b4..f2ad3c1ee466 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
index f1f5a01a89b6..9bd46c432fca 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
@@ -36,7 +36,7 @@ export interface ReadOnlyFirst {
 /**
  * Check if a given object implements the ReadOnlyFirst interface.
  */
-export function instanceOfReadOnlyFirst(value: object): boolean {
+export function instanceOfReadOnlyFirst(value: object): value is ReadOnlyFirst {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
index 68b15542cbbd..3de8fb2d62be 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
@@ -30,7 +30,7 @@ export interface Return {
 /**
  * Check if a given object implements the Return interface.
  */
-export function instanceOfReturn(value: object): boolean {
+export function instanceOfReturn(value: object): value is Return {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
index b302e6f03627..34edcaa2f004 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
@@ -30,7 +30,7 @@ export interface SpecialModelName {
 /**
  * Check if a given object implements the SpecialModelName interface.
  */
-export function instanceOfSpecialModelName(value: object): boolean {
+export function instanceOfSpecialModelName(value: object): value is SpecialModelName {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
index 4ae4af399fd8..f0ecd59d5ba2 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
index c85ea3d6ed63..2028044d626c 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
index 19e40eaf4b86..54f256e339f5 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
@@ -36,7 +36,7 @@ export interface Category {
 /**
  * Check if a given object implements the Category interface.
  */
-export function instanceOfCategory(value: object): boolean {
+export function instanceOfCategory(value: object): value is Category {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
index 4496efe73f05..bc250bd002cb 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
@@ -42,7 +42,7 @@ export interface ModelApiResponse {
 /**
  * Check if a given object implements the ModelApiResponse interface.
  */
-export function instanceOfModelApiResponse(value: object): boolean {
+export function instanceOfModelApiResponse(value: object): value is ModelApiResponse {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
index 948cda435c8a..f1a3a4bb0f0a 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
@@ -72,7 +72,7 @@ export type OrderStatusEnum = typeof OrderStatusEnum[keyof typeof OrderStatusEnu
 /**
  * Check if a given object implements the Order interface.
  */
-export function instanceOfOrder(value: object): boolean {
+export function instanceOfOrder(value: object): value is Order {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
index 3c60d980598b..476b11aa0efc 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
@@ -85,9 +85,9 @@ export type PetStatusEnum = typeof PetStatusEnum[keyof typeof PetStatusEnum];
 /**
  * Check if a given object implements the Pet interface.
  */
-export function instanceOfPet(value: object): boolean {
-    if (!('name' in value)) return false;
-    if (!('photoUrls' in value)) return false;
+export function instanceOfPet(value: object): value is Pet {
+    if (!('name' in value) || value['name'] === undefined) return false;
+    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
index 0a5bdbdde07c..fd877416ade1 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
@@ -36,7 +36,7 @@ export interface Tag {
 /**
  * Check if a given object implements the Tag interface.
  */
-export function instanceOfTag(value: object): boolean {
+export function instanceOfTag(value: object): value is Tag {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
index 3924497622f9..149be73b9ffc 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
@@ -72,7 +72,7 @@ export interface User {
 /**
  * Check if a given object implements the User interface.
  */
-export function instanceOfUser(value: object): boolean {
+export function instanceOfUser(value: object): value is User {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
index a902fb9c9af3..8f61f66faaa1 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
@@ -61,7 +61,7 @@ export interface EnumPatternObject {
 /**
  * Check if a given object implements the EnumPatternObject interface.
  */
-export function instanceOfEnumPatternObject(value: object): boolean {
+export function instanceOfEnumPatternObject(value: object): value is EnumPatternObject {
     return true;
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
index a801fcf5ef18..fe388268b172 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
@@ -86,7 +86,7 @@ export enum FakeEnumRequestGetInline200ResponseNullableNumberEnumEnum {
 /**
  * Check if a given object implements the FakeEnumRequestGetInline200Response interface.
  */
-export function instanceOfFakeEnumRequestGetInline200Response(value: object): boolean {
+export function instanceOfFakeEnumRequestGetInline200Response(value: object): value is FakeEnumRequestGetInline200Response {
     return true;
 }