Handle functions & methods with implicit return types.

^KT-78422

Author: aviroop-123

compiler/fir/analysis-tests/testData/resolve/headerMode/classDeclaration.fir.txt

@@ -19,6 +19,8 @@ FILE: classDeclaration.kt
 
         private final fun funC(): R|kotlin/String|
 
+        public final fun funD(): R|kotlin/Int|
+
     }
     public abstract interface B : R|kotlin/Any| {
         public open fun funA(): R|kotlin/String|

compiler/fir/analysis-tests/testData/resolve/headerMode/classDeclaration.kt

@@ -23,6 +23,8 @@ class A {
     private fun funC(): String {
         return "A.funC body"
     }
+
+    fun funD() = 1 + 2
 }
 
 interface B {

compiler/fir/analysis-tests/testData/resolve/headerMode/functionDeclaration.fir.txt

@@ -10,3 +10,4 @@ FILE: functionDeclaration.kt
         >
 
     private final fun funC(): R|kotlin/String|
+    public final fun funD(): R|kotlin/Int|

compiler/fir/analysis-tests/testData/resolve/headerMode/functionDeclaration.kt

@@ -23,5 +23,7 @@ private fun funC(): String {
     return "funC body"
 }
 
+fun funD() = 1 + 2
+
 /* GENERATED_FIR_TAGS: classReference, contractConditionalEffect, contracts, functionDeclaration, inline, nullableType,
 stringLiteral */

compiler/fir/analysis-tests/testData/resolve/headerMode/objectDeclaration.fir.txt

@@ -19,4 +19,6 @@ FILE: objectDeclaration.kt
 
         private final fun funC(): R|kotlin/String|
 
+        public final fun funD(): R|kotlin/Int|
+
     }

compiler/fir/analysis-tests/testData/resolve/headerMode/objectDeclaration.kt

@@ -23,6 +23,8 @@ object A {
     private fun funC(): String {
         return "A.funC body"
     }
+
+    fun funD() = 1 + 2
 }
 
 /* GENERATED_FIR_TAGS: classReference, contractConditionalEffect, contracts, functionDeclaration, inline, nullableType,

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/LightTreeRawFirDeclarationBuilder.kt

@@ -2087,7 +2087,12 @@ class LightTreeRawFirDeclarationBuilder(
 
                     val allowLegacyContractDescription = outerContractDescription == null
                     val bodyWithContractDescription = withForcedLocalContext {
-                        convertFunctionBody(block, expression, allowLegacyContractDescription, headerMode)
+                        convertFunctionBody(
+                            block,
+                            expression,
+                            allowLegacyContractDescription,
+                            headerMode && (returnTypeRef !is FirImplicitTypeRef)
+                        )
                     }
                     this.body = bodyWithContractDescription.first
                     val contractDescription = outerContractDescription ?: bodyWithContractDescription.second

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirStatusResolveTransformer.kt

@@ -19,6 +19,7 @@ import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.resolve.transformers.body.resolve.LocalClassesNavigationInfo
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.symbols.lazyResolveToPhase
+import org.jetbrains.kotlin.fir.types.FirImplicitTypeRef
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.types.coneType
 import org.jetbrains.kotlin.fir.utils.exceptions.withFirEntry
@@ -428,7 +429,7 @@ abstract class AbstractFirStatusResolveTransformer(
             overriddenFunctions.map { it.status as FirResolvedDeclarationStatus },
         )
         // Once the modality is determined, we can remove the body.
-        if (simpleFunction.isHeader == true) simpleFunction.replaceBody(null)
+        if (simpleFunction.isHeader == true && simpleFunction.returnTypeRef !is FirImplicitTypeRef) simpleFunction.replaceBody(null)
 
         simpleFunction.transformStatus(this, resolvedStatus)
         transformDeclaration(simpleFunction, data) as FirStatement

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt

@@ -21,6 +21,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyBackingField
 import org.jetbrains.kotlin.fir.declarations.synthetic.FirSyntheticProperty
 import org.jetbrains.kotlin.fir.declarations.utils.hasExplicitBackingField
 import org.jetbrains.kotlin.fir.declarations.utils.isConst
+import org.jetbrains.kotlin.fir.declarations.utils.isHeader
 import org.jetbrains.kotlin.fir.declarations.utils.isInline
 import org.jetbrains.kotlin.fir.declarations.utils.isLocal
 import org.jetbrains.kotlin.fir.declarations.utils.isScriptTopLevelDeclaration
@@ -1038,6 +1039,8 @@ open class FirDeclarationsResolveTransformer(
                 }
             result.transformReturnTypeRef(transformer, ResolutionMode.UpdateImplicitTypeRef(returnTypeRef))
         }
+        // Once the return type is resolved, the body can be removed.
+        if (result.isHeader == true) result.replaceBody(null)
 
         return result
     }