Standard Library Modules: error C2039: `'promise_type'`: is not a member of `'std::coroutine_traits<TestModule::FireAndForget>'`

[StephanTLavavej]

Repros with VS 2022 17.11 Preview 3. Reported by CppMan on Discord.

I'm not familiar enough with coroutines, or with multiple module scenarios, to say what should happen when a module tries to export a partial specialization like std::coroutine_traits<TestModule::FireAndForget, Ts...>.

C:\Temp>type meow.ixx

export module Module;

import std;

export namespace TestModule {
    struct FireAndForget {};
} // namespace TestModule

export template <typename... Ts>
struct std::coroutine_traits<TestModule::FireAndForget, Ts...> {
    struct promise_type {
        TestModule::FireAndForget get_return_object() const noexcept {
            return {};
        }

        void return_void() const noexcept {}

        std::suspend_never initial_suspend() const noexcept {
            return {};
        }

        std::suspend_never final_suspend() const noexcept {
            return {};
        }

        void unhandled_exception() const noexcept {}
    };
};

C:\Temp>type test.cpp

import Module;
import std;

TestModule::FireAndForget Test() {
    co_return;
}

int main() {
    Test();
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c "%VCToolsInstallDir%\modules\std.ixx"
std.ixx

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c meow.ixx
meow.ixx

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od test.cpp meow.obj std.obj
test.cpp
test.cpp(5): error C2039: 'promise_type': is not a member of 'std::coroutine_traits<TestModule::FireAndForget>'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.41.34021\include\coroutine(46): note: see declaration of 'std::coroutine_traits<TestModule::FireAndForget>'

[cdacamar]

Yes, I can confirm this is a compiler bug. There is a workaround:

meow.ixx:

export module Module;

import std;

export namespace TestModule {
    struct FireAndForget {};
} // namespace TestModule

namespace std { // Note: we remove the qualified name declaration.
export template <typename... Ts>
struct coroutine_traits<TestModule::FireAndForget, Ts...> {
    struct promise_type {
        TestModule::FireAndForget get_return_object() const noexcept {
            return {};
        }

        void return_void() const noexcept {}

        std::suspend_never initial_suspend() const noexcept {
            return {};
        }

        std::suspend_never final_suspend() const noexcept {
            return {};
        }

        void unhandled_exception() const noexcept {}
    };
};
}

[naddu77]

I am CppMan.
The same issue occurs with the std::formatter class template specialization.
The same can be solved by modifying as Cameron DaCamara (@cdacamar) suggested.
Thank you.

[StephanTLavavej]

Cameron DaCamara (@cdacamar) fixed this with internal MSVC-PR-565655 which will be available in VS 2022 17.12 Preview 2. Thanks again for the report!