Hmmm... thinking about it, is this possible?
1. Remove the constraints to match anything.
2. Inside the template, have some construct that enumerates all
possible overloads and UFCS functions that are visible at the
point of instantiation.
3. If this set contains only the current template, use a static
assert to print the message.
4. Otherwise, make the template fail to compile somehow (for
example, evaluate a semantically invalid expression), and hope
that the compiler will then take the other overloads into
consideration (SFINAE).
Could this work?
