Funny code

This is to enforce that the first parameter is a uint64_t (or more
precisely the underlying type) and nothing which is convertible to
an uin64_t.
void fn( same_as<uint64_t> auto param )
{
}
template void fn( uint64_t param );
The instantiation is to place the code not in a header completely.
The funny thing about that is that the definition doesn't look
generic but the instantiation does since there's a template clause.