[Bug c++/115596] New: Default arguments are erroneously allowed for parameters of a requires expression

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115596

            Bug ID: 115596
           Summary: Default arguments are erroneously allowed for
                    parameters of a requires expression
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

GCC accepts invalid requires expression as follows with default arguments of a
parameter:
```
template<typename T>
concept C = requires(T t = {}) {
    t;
};
```
Both Clang and MSVC reject it.

Online demo: https://gcc.godbolt.org/z/EMncoT8Yv