[c++][Modules] Member variable of non-exported type has inaccessible hidden friend with defaulted comparison inside namespace

[davidstone]

The following two valid translation units

export module a;

namespace n {

struct monostate {
	friend auto operator==(monostate, monostate) -> bool = default;
};

export struct a {
	friend auto operator==(a, a) -> bool = default;
	monostate m;
};

} // namespace n

export module b;

import a;

namespace n {

auto b() -> void {
	a() == a();
}

} // namespace n

causes clang to fail to compile with

In file included from /app/b.cpp:3:
a.cpp:10:47: error: invalid operands to binary expression ('monostate' and 'monostate')
   10 |         friend auto operator==(a, a) -> bool = default;
      |                                                      ^
b.cpp:8:6: note: in defaulted equality comparison operator for 'a' first required here
    8 |         a() == a();
      |             ^
b.cpp:8:6: warning: equality comparison result unused [-Wunused-comparison]
    8 |         a() == a();
      |         ~~~~^~~~~~
1 warning and 1 error generated.

See it live: https://godbolt.org/z/TvWs7osPs

This is basically the same as #123815 except instead of qualifying the call site with the namespace of the outer type, we are calling from within the namespace. This feels somewhat similar to but not the same as #133720

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: David Stone (davidstone)

The following two valid translation units

export module a;

namespace n {

struct monostate {
	friend auto operator==(monostate, monostate) -> bool = default;
};

export struct a {
	friend auto operator==(a, a) -> bool = default;
	monostate m;
};

} // namespace n

export module b;

import a;

namespace n {

auto b() -> void {
	a() == a();
}

} // namespace n

causes clang to fail to compile with

In file included from /app/b.cpp:3:
a.cpp:10:47: error: invalid operands to binary expression ('monostate' and 'monostate')
   10 |         friend auto operator==(a, a) -> bool = default;
      |                                                      ^
b.cpp:8:6: note: in defaulted equality comparison operator for 'a' first required here
    8 |         a() == a();
      |             ^
b.cpp:8:6: warning: equality comparison result unused [-Wunused-comparison]
    8 |         a() == a();
      |         ~~~~^~~~~~
1 warning and 1 error generated.

See it live: https://godbolt.org/z/TvWs7osPs

This is basically the same as #123815 except instead of qualifying the call site with the namespace of the outer type, we are calling from within the namespace. I suspect this is also the same issue as #133720

[davidstone]

Note that explicitly qualifying the type with n::a() == n::a() does not solve the issue. It is the fact that the call is inside the namespace n that causes the bug.