[interop] C++ extension for class template specialization isn't visible when forward declaration for it is present

[hyp]

Swift fails to compile a file as it can't find a method in an extension for a C++ class template, when this template specialization typealias is in module B, and module A defines it and has a forward declaration for this template.

Steps to reproduce:
$ cat test.h

#pragma once

template<class T>
struct TestStruct;

TestStruct<int> returnStruct();

template<class T>
struct TestStruct {
    int xy;
};

$ cat test3.h

#pragma once
#include "test.h"

using TestStructInt = TestStruct<int>;

$ cat module.modulemap

module Testi {
  header "test.h"
  requires cplusplus
}

module Testi3 {
  header "test3.h"
  requires cplusplus
}

$ cat test.swift

import Testi
import Testi3

extension TestStructInt {

public func testExtensionFunc() {
  print("Hey, this is extension speaking")
}

}

$ cat test2.swift

import Testi
import Testi3
import TestExt

func test() {
   let t = TestStructInt()
   t.testExtensionFunc()
}

invoke:
$ bin/swiftc -sdk $SDK -emit-module -module-name TestExt test.swift -I . -Xfrontend -enable-experimental-cxx-interop
$ bin/swiftc -sdk $SDK -emit-module -module-name TestProd test2.swift -I . -Xfrontend -enable-experimental-cxx-interop

RESULT:

test2.swift:8:6: error: value of type 'TestStructInt' (aka '__CxxTemplateInst10TestStructIiE') has no member 'testExtensionFunc'
   t.testExtensionFunc()
   ~ ^~~~~~~~~~~~~~~~

Expected result:

Should compile.

If I remove the forward declaration from test.h, it works.