[Bug c++/114946] New: [concepts] No error for invalid requires constraint in declaration

[nshead at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114946

            Bug ID: 114946
           Summary: [concepts] No error for invalid requires constraint in
                    declaration
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nshead at gcc dot gnu.org
  Target Milestone: ---

The following sample compiles fine with 'g++ -std=c++20 -pedantic-errors':

  template <typename T>
    requires xxxx
  struct S {};

  template <typename T>
    requires xxxx
  void foo() {}

Note that 'xxxx' has not been declared or defined.  Both MSVC and Clang
complain about the undeclared identifier.  GCC does error if we attempt to
instantiate either of these specialisations, but they always (silently) lose to
a better match:

  template <typename T> struct S {};
  template <typename T> requires xxxx struct S<T> {};

  template <typename T> void foo() {}
  template <typename T> requires xxxx void foo() {}

  int main() {
    S<int> x;
    foo<int>();
  }