From 9d7a04277ebb5edcfe649977fb5bb45ac91573f6 Mon Sep 17 00:00:00 2001
From: Jukka Lehtosalo <jukka.lehtosalo@iki.fi>
Date: Wed, 18 Sep 2024 14:42:25 +0100
Subject: [PATCH] [PEP 695] Fix covariance of frozen dataclasses (#17783)

Fix this conformance test:
```
@dataclass(frozen=True)
class ShouldBeCovariant4[T]:
    x: T

vo4_1: ShouldBeCovariant4[float] = ShouldBeCovariant4[int](1)  # OK
vo4_2: ShouldBeCovariant4[int] = ShouldBeCovariant4[float](1)  # E
```
Link:

https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L66
---
 mypy/subtypes.py                    |  3 ++-
 test-data/unit/check-python312.test | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index 6e2366c4e0df..5c4471cc5b62 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -2004,7 +2004,8 @@ def infer_variance(info: TypeInfo, i: int) -> bool:
         tvar = info.defn.type_vars[i]
         self_type = fill_typevars(info)
         for member in all_non_object_members(info):
-            if member in ("__init__", "__new__"):
+            # __mypy-replace is an implementation detail of the dataclass plugin
+            if member in ("__init__", "__new__", "__mypy-replace"):
                 continue
 
             if isinstance(self_type, TupleType):
diff --git a/test-data/unit/check-python312.test b/test-data/unit/check-python312.test
index 9dc52d2c07b0..d0a39f7e56a6 100644
--- a/test-data/unit/check-python312.test
+++ b/test-data/unit/check-python312.test
@@ -249,6 +249,25 @@ if int():
 if int():
     f = e
 
+[case testPEP695InferVarianceInFrozenDataclass]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class Covariant[T]:
+    x: T
+
+cov1: Covariant[float] = Covariant[int](1)
+cov2: Covariant[int] = Covariant[float](1)  # E: Incompatible types in assignment (expression has type "Covariant[float]", variable has type "Covariant[int]")
+
+@dataclass(frozen=True)
+class Invariant[T]:
+    x: list[T]
+
+inv1: Invariant[float] = Invariant[int]([1])  # E: Incompatible types in assignment (expression has type "Invariant[int]", variable has type "Invariant[float]")
+inv2: Invariant[int] = Invariant[float]([1])  # E: Incompatible types in assignment (expression has type "Invariant[float]", variable has type "Invariant[int]")
+[builtins fixtures/tuple.pyi]
+
 [case testPEP695InferVarianceCalculateOnDemand]
 # flags: --enable-incomplete-feature=NewGenericSyntax