template isThing( alias symbol, A )
{
  enum isThing = false;
}

This template works in most contexts:

int x;
struct S {}

pragma(msg, isThing!x);
pragma(msg, isThing!S);

But this fails:
pragma(msg, isThing!int);

Why does it fail on a basic type (int), but not a user defined type (S)?
How can I fix the template declaration to not error in that case?
I tried:

template isThing( alias symbol, A )
{
  enum isThing = false;
}
template isThing( T, A )
{
  enum isThing = false;
}

Hoping the T version would catch the int, but this leads to different errors "matches more than one template declaration".