[Bug c++/57891] No diagnostic of narrowing conversion in non-type template argument

[tadeus.prastowo at unitn dot it]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57891

--- Comment #9 from Tadeus Prastowo <tadeus.prastowo at unitn dot it> ---
This problem still exists in the trunk (cf. https://godbolt.org/g/bRf18i). 
Clang correctly keeps rejecting it (cf. https://godbolt.org/g/egcNtV).  Both
use the following MWE:

template<unsigned a>
struct X {
  static constexpr unsigned data {a};
};

int main() {
  return X<-1>::data;
}