[Bug c++/94922] Functional cast from base class to derived class incorrectly accepted with -std=c++2a

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94922

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The above appears to be a 10/11 regression, but note that all versions of GCC
>= 4.7 also incorrectly accept the following testcase:

struct base {};
struct derived : base {};

template <typename = decltype(derived(base()))>
void foo();

void
bar()
{
  foo();
}