https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100396
--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to vopl from comment #2)
> Please, try this, also failed
>
> /////////0/////////1/////////2/////////3/////////4/////////5/////////6///////
> //7
> template<class F, class... Args> struct Checker
> {
> using Some = decltype(F{}(Args{}...));
> };
>
> template<class F, class... Args> concept valid =
> requires { typename Checker<F, Args...>::Some; };
>
> /////////0/////////1/////////2/////////3/////////4/////////5/////////6///////
> //7
> template<class F, class... Args> auto ovr(F f, Args... args)
> requires valid<F, Args...>
> {
> return ovr<F, Args...>(int{}, f, args...); // a second ovr is expected
> to be called, but the compiler tries to call this ovr again. Collision
> between 'int' and 'F' is ignored, extra Args provided to call
> }
>
> template<class F, class... Args> auto ovr(int, F f, Args... args)
> {
> return f(args...);
> }
>
> /////////0/////////1/////////2/////////3/////////4/////////5/////////6///////
> //7
> void use()
> {
> ovr([]{});
> }
>
>
> $ g++-11.1.0 -std=c++20 -c src.cpp
> src.cpp: In instantiation of 'struct Checker<use()::<lambda()>,
> use()::<lambda()> >':
> src.cpp:8:25: required by substitution of 'template<class F, class ...
> Args> auto ovr(F, Args ...) requires valid<F, Args ...> [with F =
> use()::<lambda()>; Args = {use()::<lambda()>}]'
> src.cpp:14:27: required from 'auto ovr(F, Args ...) requires valid<F,
> Args ...> [with F = use()::<lambda()>; Args = {}]'
> src.cpp:25:8: required from here
> src.cpp:4:30: error: no match for call to '(use()::<lambda()>)
> (use()::<lambda()>)'
> 4 | using Some = decltype(F{}(Args{}...));
> | ~~~^~~~~~~~~~~
> src.cpp:4:30: note: candidate: 'void (*)()' (conversion)
> src.cpp:4:30: note: candidate expects 1 argument, 2 provided
> src.cpp:25:9: note: candidate: 'use()::<lambda()>'
> 25 | ovr([]{});
> | ^
> src.cpp:25:9: note: candidate expects 0 arguments, 1 provided
>
> ----------
> IMHO, the problem is not caused by concepts, but caused by corrupted
> overloading selection mechanic. Please, take a look at the collision between
> argument 'int{}' and first template parameter specified as 'F'. Next, no
> SFINAE expected in this case due to correct overloading, but overloading
> failed and SFINAE happens. Thanks.
Although the collision between 'int{}' and the template argument for 'F' does
mean that the first overload ultimately isn't viable, the compiler as per
CWG2369 first needs to check the overload's constraints (using the
provided/deduced template arguments) _before_ checking whether 'int' is
convertible to 'F', and it's during this constraint checking we hit the hard
error. So as far as I can tell, the compiler is behaving as specified. See
also PR99599 and its related PRs.
