[FE] Substitute fixed type variables with inferred stub types

Actually, a type variable might be fixed into a stub type. Such stub type should be substituted before sub calls completion

^KT-51988 Fixed

compiler/frontend/src/org/jetbrains/kotlin/resolve/DelegateInferenceSession.kt

@@ -97,7 +97,7 @@ class DelegateInferenceSession(
 
     override fun inferPostponedVariables(
         lambda: ResolvedLambdaAtom,
-        initialStorage: ConstraintStorage,
+        constraintSystemBuilder: ConstraintSystemBuilder,
         completionMode: ConstraintSystemCompletionMode,
         diagnosticsHolder: KotlinDiagnosticsHolder
     ): Map<TypeConstructor, UnwrappedType> = emptyMap()
@@ -124,7 +124,7 @@ class InferenceSessionForExistingCandidates(
     override fun currentConstraintSystem(): ConstraintStorage = ConstraintStorage.Empty
     override fun inferPostponedVariables(
         lambda: ResolvedLambdaAtom,
-        initialStorage: ConstraintStorage,
+        constraintSystemBuilder: ConstraintSystemBuilder,
         completionMode: ConstraintSystemCompletionMode,
         diagnosticsHolder: KotlinDiagnosticsHolder
     ): Map<TypeConstructor, UnwrappedType> = emptyMap()

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/BuilderInferenceSession.kt

@@ -233,22 +233,23 @@ class BuilderInferenceSession(
 
     override fun inferPostponedVariables(
         lambda: ResolvedLambdaAtom,
-        initialStorage: ConstraintStorage,
+        constraintSystemBuilder: ConstraintSystemBuilder,
         completionMode: ConstraintSystemCompletionMode,
         diagnosticsHolder: KotlinDiagnosticsHolder,
     ): Map<TypeConstructor, UnwrappedType>? {
         initializeLambda(lambda)
 
-        fun getResultingSubstitutor(): NewTypeSubstitutor {
+        val initialStorage = constraintSystemBuilder.currentStorage()
+        val resultingSubstitutor by lazy {
             val storageSubstitutor = initialStorage.buildResultingSubstitutor(commonSystem, transformTypeVariablesToErrorTypes = false)
-            return ComposedSubstitutor(storageSubstitutor, commonSystem.buildCurrentSubstitutor() as NewTypeSubstitutor)
+            ComposedSubstitutor(storageSubstitutor, commonSystem.buildCurrentSubstitutor() as NewTypeSubstitutor)
         }
 
         val effectivelyEmptyConstraintSystem = initializeCommonSystem(initialStorage)
 
         if (effectivelyEmptyConstraintSystem) {
             if (isTopLevelBuilderInferenceCall()) {
-                updateAllCalls(getResultingSubstitutor())
+                updateAllCalls(resultingSubstitutor)
             }
             return null
         }
@@ -261,8 +262,14 @@ class BuilderInferenceSession(
             diagnosticsHolder
         )
 
+        if (completionMode == ConstraintSystemCompletionMode.FULL) {
+            constraintSystemBuilder.substituteFixedVariables(
+                ComposedSubstitutor(resultingSubstitutor, createNonFixedTypeToVariableSubstitutor())
+            )
+        }
+
         if (isTopLevelBuilderInferenceCall()) {
-            updateAllCalls(getResultingSubstitutor())
+            updateAllCalls(resultingSubstitutor)
         }
 
         return commonSystem.fixedTypeVariables.cast() // TODO: SUB