[Bug c++/96213] GCC doesn't complain about ill-formed non-dependent template default argument

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96213

--- Comment #2 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
Here's a similar situation I just ran into again, somehow!

// https://godbolt.org/z/3TKG1z
struct S {};
template<class = decltype(S(42), void())> void f() {}
template<int I=42, class = decltype(S(I), void())> void g() {}
int main() {
    f();  // correctly errors out
    g();  // incorrectly accepted by GCC
}

GCC is happy to treat `S(I)` as well-formed when `I` is a template parameter.
Obviously `decltype(S(I), void())` can't be anything because `S(I)` is
ill-formed, so I'm not sure what GCC is doing to compile this.