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From: "vopl at bk dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100396] [11.1 regression] The template function overload is
 not selected correctly
Date: Mon, 03 May 2021 16:07:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100396

--- 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 =3D decltype(F{}(Args{}...));
};

template<class F, class... Args> concept valid =3D
    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=3Dc++20 -c src.cpp=20
src.cpp: In instantiation of 'struct Checker<use()::<lambda()>,
use()::<lambda()> >':
src.cpp:8:25:   required by substitution of 'template<class F, class ... Ar=
gs>
auto ovr(F, Args ...) requires  valid<F, Args ...> [with F =3D use()::<lamb=
da()>;
Args =3D {use()::<lambda()>}]'
src.cpp:14:27:   required from 'auto ovr(F, Args ...) requires  valid<F, Ar=
gs
...> [with F =3D use()::<lambda()>; Args =3D {}]'
src.cpp:25:8:   required from here
src.cpp:4:30: error: no match for call to '(use()::<lambda()>)
(use()::<lambda()>)'
    4 |     using Some =3D 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 SF=
INAE
expected in this case due to correct overloading, but overloading failed and
SFINAE happens. Thanks.=