[Bug c++/99518] New: trailing requires clause adds nonexistent reference declarator to function parameter

["nickgray0 at brown dot edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99518

            Bug ID: 99518
           Summary: trailing requires clause adds nonexistent reference
                    declarator to function parameter
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickgray0 at brown dot edu
  Target Milestone: ---

the following code fails to compile

    #include <concepts>

    auto f(std::integral auto x) {}
    auto g(auto x) requires requires { { x }->std::integral; } {}

    auto main()->int {
        f(42); // OK
        g(42); // error
    }



error message: In function 'int main()':
<source>:8:9: error: no matching function for call to 'g(int)'
    8 |     g(42); // error
      |         ^
<source>:4:6: note: candidate: 'template<class auto:2> auto g(auto:2) requires
requires{{g::x} -> decltype(auto) [requires std::integral<<placeholder>, >];}'
    4 | auto g(auto x) requires requires { { x }->std::integral; } {}
      |      ^
<source>:4:6: note:   template argument deduction/substitution failed:
<source>:4:6: note: constraints not satisfied
<source>: In substitution of 'template<class auto:2> auto g(auto:2) requires
requires{{x} -> decltype(auto) [requires std::integral<<placeholder>, >];}
[with auto:2 = int]':
<source>:8:9:   required from here
<source>:4:6:   required by the constraints of 'template<class auto:2> auto
g(auto:2) requires requires{{x} -> decltype(auto) [requires
std::integral<<placeholder>, >];}'
<source>:4:25:   in requirements  [with auto:2 = int]
<source>:4:38: note: 'x' does not satisfy return-type-requirement, because
    4 | auto g(auto x) requires requires { { x }->std::integral; } {}
      |                                      ^
<source>:4:38: error: deduced expression type does not satisfy placeholder
constraints
    4 | auto g(auto x) requires requires { { x }->std::integral; } {}
      |                                    ~~^~~~~~~~~~~~~~~~~~
<source>:4:38: note: constraints not satisfied
In file included from <source>:1:
/opt/compiler-explorer/gcc-trunk-20210310/include/c++/11.0.1/concepts:102:13:  
required for the satisfaction of 'integral<decltype(auto) [requires
std::integral<<placeholder>, >]>' [with decltype(auto) [requires
std::integral<<placeholder>, >] = int&]
/opt/compiler-explorer/gcc-trunk-20210310/include/c++/11.0.1/concepts:102:24:
note: the expression 'is_integral_v<_Tp> [with _Tp = int&]' evaluated to
'false'
  102 |     concept integral = is_integral_v<_Tp>;