[Bug c++/89442] New: [concepts] missing "wrong number of template arguments" error in requires-clause

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89442

            Bug ID: 89442
           Summary: [concepts] missing "wrong number of template
                    arguments" error in requires-clause
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<typename T, typename U> constexpr bool foo = true;

template<typename T> int f(T) requires foo<T> { return 0; }

int i = f(0);



$ g++17 -fconcepts con.cc -c
con.cc:5:12: error: cannot call function 'int f(T) requires  foo<T> [with T =
int]'
    5 | int i = f(0);
      |            ^
con.cc:3:26: note:   constraints not satisfied
    3 | template<typename T> int f(T) requires foo<T> { return 0; }
      |                          ^
con.cc:3:26: note: 'foo<T>' evaluated to false


The error tells me that foo<T> evaluated to false, but it should say foo<T> is
invalid and report an ill-formed constraint, as in this equivalent version:

template<typename, typename>
struct bar { static constexpr bool value = true; };

template<typename T> int f(T) requires bar<T>::value { return 0; }

int i = f(0);

This second one (correctly) says:

con.cc:4:45: error: wrong number of template arguments (1, should be 2)
    4 | template<typename T> int f(T) requires bar<T>::value { return 0; }
      |                                             ^
con.cc:2:8: note: provided for 'template<class, class> struct bar'
    2 | struct bar { static constexpr bool value = true; };
      |        ^~~
con.cc:6:12: error: cannot call function 'int f(T) requires 
<erroneous-expression> [with T = int]'
    6 | int i = f(0);
      |            ^
con.cc:4:26: note:   constraints not satisfied
    4 | template<typename T> int f(T) requires bar<T>::value { return 0; }
      |                          ^
con.cc:6:12: note: ill-formed constraint
    6 | int i = f(0);
      |            ^