[Bug c++/126168] requires clause should enforce no default argument

[ncm at cantrip dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126168

ncm at cantrip dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|17.0                        |15.2.1

--- Comment #1 from ncm at cantrip dot org ---
According to [expr.prim.req.general] para 4, 
"A local parameter of a *requires-expression* shall not have a default
argument."

Gcc does not enforce this restriction:

```
template <typename T>
void f()
  requires requires(T t = T()){ t; }
{}
```

Clang says "error: default arguments are not allowed for parameters of a
requires expression"