Report 'free function called as extension' on Function subclasses

You can't make a value of some type invokable as an extension by adding
'extension' annotation to the type

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/tasks/synthesizedInvokes.kt

@@ -87,7 +87,14 @@ private fun createSynthesizedFunctionWithFirstParameterAsReceiver(descriptor: Fu
 }
 
 fun isSynthesizedInvoke(descriptor: DeclarationDescriptor): Boolean {
-    return descriptor.getName() == OperatorConventions.INVOKE &&
-           (descriptor as? FunctionDescriptor)?.getKind() == CallableMemberDescriptor.Kind.SYNTHESIZED &&
-           descriptor.getContainingDeclaration() is FunctionClassDescriptor
+    if (descriptor.getName() != OperatorConventions.INVOKE || descriptor !is FunctionDescriptor) return false
+
+    var real: FunctionDescriptor = descriptor
+    while (!real.getKind().isReal()) {
+        // You can't override two different synthesized invokes at the same time
+        real = real.getOverriddenDescriptors().singleOrNull() ?: return false
+    }
+
+    return real.getKind() == CallableMemberDescriptor.Kind.SYNTHESIZED &&
+           real.getContainingDeclaration() is FunctionClassDescriptor
 }

compiler/testData/diagnostics/tests/FreeFunctionCalledAsExtension.kt

@@ -1,3 +1,13 @@
 fun foo(a: (String) -> Unit) {
     "".<!FREE_FUNCTION_CALLED_AS_EXTENSION!>a<!>()
 }
+
+
+
+interface A : (String) -> Unit {}
+
+fun foo(a: @extension A) {
+    // @extension annotation on an unrelated type shouldn't have any effect on this diagnostic.
+    // Only kotlin.Function{n} type annotated with @extension should
+    "".<!FREE_FUNCTION_CALLED_AS_EXTENSION!>a<!>()
+}

compiler/testData/diagnostics/tests/FreeFunctionCalledAsExtension.txt

@@ -1,3 +1,11 @@
 package
 
 internal fun foo(/*0*/ a: (kotlin.String) -> kotlin.Unit): kotlin.Unit
+internal fun foo(/*0*/ a: @[kotlin.extension()] A): kotlin.Unit
+
+internal interface A : (kotlin.String) -> kotlin.Unit {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(/*0*/ p1: kotlin.String): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}