[Bug libstdc++/93795] New: Out-of-line definition of class member template with 'requires std::invocable' constraint fails to compile

[plexando at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93795

            Bug ID: 93795
           Summary: Out-of-line definition of class member template with
                    'requires std::invocable<T>' constraint fails to
                    compile
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: plexando at protonmail dot com
  Target Milestone: ---

A class member template having a constraint with a 'requires std::invocable<T>'
clause fails to compile if defined out-of-line.

For instance, the program

#include <concepts>
#include <iostream>

struct S {
    template<typename T> requires std::invocable<T>
    void f(T&& t);
};

template<typename T> requires std::invocable<T>
void S::f(T&& t) { t(); }

int main() {
    S{}.f([] { std::cerr << -1 << std::endl; });
}

does not compile with GCC 10.0.1 20200214 (experimental) on x86_64 GNU/Linux
(It does compile however if the member template is defined in-line).

Compiler output (using g++ prog.cc -Wall -std=gnu++2a) is:

prog.cc:13:1: error: no declaration matches 'void S::f(T&&)'
   13 | S::f(
      | ^
prog.cc:7:10: note: candidate is: 'template<class T>  requires  invocable<T>
void S::f(T&&)'
    7 |     void f(T&& t);
      |          ^
prog.cc:4:8: note: 'struct S' defined here
    4 | struct S {
      |        ^
prog.cc:7:10: error: 'void S::f(T&&) [with T = main()::<lambda()>]', declared
using local type 'main()::<lambda()>', is used but never defined [-fpermissive]
    7 |     void f(T&& t);
      |          ^
prog.cc:7:10: warning: 'void S::f(T&&) [with T = main()::<lambda()>]' used but
never defined

Configure options were ./configure --prefix=/usr/local/gcc-10-20200209.