https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100396
--- Comment #4 from vopl at bk dot ru ---
I understand what you're saying. But, apparently, there is still corruption in
the calculus of the substituted Args, please look at this code (thanks to
johny5):
/////////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...);
}
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([](double){}, 2.);
}
-----------------------
<source>: In instantiation of 'struct Checker<use()::<lambda(double)>, double,
double>':
<source>:8:25: required by substitution of 'template<class F, class ... Args>
auto ovr(F, Args ...) requires valid<F, Args ...> [with F =
use()::<lambda(double)>; Args = {double, double}]'
<source>:14:27: required from 'auto ovr(F, Args ...) requires valid<F, Args
...> [with F = use()::<lambda(double)>; Args = {double}]'
<source>:25:8: required from here
<source>:4:30: error: no match for call to '(use()::<lambda(double)>) (double,
double)'
4 | using Some = decltype(F{}(Args{}...));
| ~~~^~~~~~~~~~~
<source>:4:30: note: candidate: 'void (*)(double)' (conversion)
<source>:4:30: note: candidate expects 2 arguments, 3 provided
<source>:25:9: note: candidate: 'use()::<lambda(double)>'
25 | ovr([](double){}, 2.);
| ^
<source>:25:9: note: candidate expects 1 argument, 2 provided
Take attention at 'Args' - it was {double} initially, but becomes {double,
double} for second ovr call
