How can I tell if the give parameter can be run at compile time?

[Jack via Digitalmars-d-learn]

bool g(T)(T)
{
        return __traits(compiles, mixin("{ enum a = t; }"));
}


int a;
enum s = "";
// both return false but g(s) is expected to return true
pragma(msg, g(s));
pragma(msg, g(a));