FIR: Make constraint injector treat stub types as type variables

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -100,6 +100,10 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         return this is StubTypeForBuilderInference || isDefNotNullStubType<StubTypeForBuilderInference>()
     }
 
+    override fun TypeConstructorMarker.unwrapStubTypeVariableConstructor(): TypeConstructorMarker {
+        return this
+    }
+
     override fun StubTypeMarker.getOriginalTypeVariable(): TypeVariableTypeConstructorMarker {
         require(this is AbstractStubType, this::errorMessage)
         return this.originalTypeVariable as TypeVariableTypeConstructorMarker