[Bug c++/113405] New: Can't access member type alias of concept-constrained class template specialization in global module fragment via alias template in different module

[eddiejnolan at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113405

            Bug ID: 113405
           Summary: Can't access member type alias of concept-constrained
                    class template specialization in global module
                    fragment via alias template in different module
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eddiejnolan at gmail dot com
  Target Milestone: ---

This example shows GCC failing to resolve the member type alias "foobar" in a
concept-constrained specialization of class template "corge" when it's accessed
from module2 via an alias template in module1.

Compiler explorer link: https://godbolt.org/z/4sh6xG6P4

Minimal reproduction:

// CMakeLists.txt

cmake_minimum_required(VERSION 3.28)
project(repro CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_library(module1)
target_sources(module1
  PUBLIC
    FILE_SET cxx_modules TYPE CXX_MODULES FILES
   module1.cpp
)
target_include_directories(module1 PRIVATE "include/")

add_library(module2)
target_sources(module2
  PUBLIC
    FILE_SET cxx_modules TYPE CXX_MODULES FILES
   module2.cpp
)
target_link_libraries(module2 PUBLIC module1)

// include/foo.h

template <typename>
concept foo = false;

template <typename>
concept bar = true;

template <typename>
struct corge {};

template <foo T>
struct corge<T> {};

template <bar T>
struct corge<T> {
  using foobar = int;
};

// module1.cpp

module;

#include "foo.h"

export module module1;

export template<class T>
using corge_alias = corge<T>::foobar;

// module2.cpp

export module module2;
import module1;

struct foobaz{};

using quux = corge_alias<foobaz>;