[Bug c++/95457] Inadequate diagnostics on constrained coroutines

[arsen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95457

Arsen Arsenović <arsen at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arsen at gcc dot gnu.org

--- Comment #1 from Arsen Arsenović <arsen at gcc dot gnu.org> ---
hm, the promise itself is not constrained, the traits specialization is.  I
imagine that might be why the diagnostic fails to manifest.  convoluting the
template a bit to make it so that there is a constraint failure on the promise
itself results in a diagnostic:

template <typename... Args>
class coroutine_traits<dummy_coroutine, Args...> {
public:
    template<typename>
    requires (... && !rvalue_reference<Args>)
    struct my_promise {
        void return_value(int x) {  }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        dummy_coroutine get_return_object() { return {}; }
        void unhandled_exception() {}
    };
    using promise_type = my_promise<void>;
};

result:
<source>: In instantiation of 'struct
std::__n4861::coroutine_traits<dummy_coroutine, int&&>':
<source>:36:15:   required from here
   36 |     co_return x;
      |               ^
<source>:26:11: error: template constraint failure for 'template<class ...
Args> template<class>  requires (... &&!(rvalue_reference<Args>)) struct
std::__n4861::coroutine_traits<dummy_coroutine, Args ...>::my_promise'
   26 |     using promise_type = my_promise<void>;
      |           ^~~~~~~~~~~~
<source>:26:11: note: constraints not satisfied
<source>: In substitution of 'template<class ... Args> template<class> 
requires (... &&!(rvalue_reference<Args>)) struct
std::__n4861::coroutine_traits<dummy_coroutine, Args ...>::my_promise [with
<template-parameter-2-1> = void; Args = {int&&}]':
<source>:26:11:   required from 'struct
std::__n4861::coroutine_traits<dummy_coroutine, int&&>'
<source>:36:15:   required from here
   36 |     co_return x;
      |               ^
<source>:19:12:   required by the constraints of 'template<class ... Args>
template<class>  requires (... &&!(rvalue_reference<Args>)) struct
std::__n4861::coroutine_traits<dummy_coroutine, Args ...>::my_promise'
<source>:18:19: note: the expression '(... &&!(rvalue_reference<Args>)) [with
Args = {int&&}]' evaluated to 'false'
   18 |     requires (... && !rvalue_reference<Args>)
      |              ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>: In function 'dummy_coroutine foo(int&&)':
<source>:36:5: error: unable to find the promise type for this coroutine
   36 |     co_return x;
      |     ^~~~~~~~~
Compiler returned: 1


unsure if this is solvable