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
