[Bug c++/105595] New: Coroutines can trigger -Wsubobject-linkage

["blubban at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105595

            Bug ID: 105595
           Summary: Coroutines can trigger -Wsubobject-linkage
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blubban at gmail dot com
  Target Milestone: ---

To reproduce:
- Create a minimal coroutine promise
- Create a coroutine where any local's type has no linkage (for example a
lambda)
- #include the above from another, differently named, file

Result:
/app/not_example.cpp:25:1: warning: 'foo()::_Z3foov.Frame' has a field
'foo()::_Z3foov.Frame::anon_ns_1_2' whose type uses the anonymous namespace
[-Wsubobject-linkage]
/app/not_example.cpp:25:1: warning: 'foo()::_Z3foov.Frame' has a field
'foo()::_Z3foov.Frame::inner_class_1_2' whose type has no linkage
[-Wsubobject-linkage]
/app/not_example.cpp:25:1: warning: 'foo()::_Z3foov.Frame' has a field
'foo()::_Z3foov.Frame::a_lambda_1_2' whose type has no linkage
[-Wsubobject-linkage]

Expected:
No warning, because the coroutine frame's type shouldn't have linkage either.
Sticking anonymous types inside each other is harmless.

Full testcase:


# 2 "/app/not_example.cpp" 1

#include <coroutine>

class async {
public:
    class promise_type {
    public:
        async get_return_object() { return {}; }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void unhandled_exception() {}
    };
};

namespace {
    class anon_ns_t {};
}

async foo()
{
    anon_ns_t anon_ns;
    class inner_class_t {} inner_class;
    auto a_lambda = [](){};
    co_await std::suspend_never{};
}


Compile with -std=c++20.

Compiler Explorer: https://godbolt.org/z/efz6dfoWq