[Bug c++/100396] [11.1 regression] The template function overload is not selected correctly

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.