[Bug c++/107168] New: Wrong errors for concepts with default lambda not-type argument

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107168

            Bug ID: 107168
           Summary: Wrong errors for concepts with default lambda not-type
                    argument
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The following code in accepted in Clang and MSVC, but GCC emits seemingly wrong
errors:

template <typename, auto = []{}> concept x = true;
struct S {
    // GCC error: template argument 2 is invalid
    operator x auto() { return 42; }
    // GCC error: wrong number of template arguments (0, should be at least 1)
    operator x<> auto() const { return 43; }
};

Online demo: https://godbolt.org/z/a3494qrG3