https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100396
--- Comment #2 from vopl at bk dot ru ---
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.
