What must a type satisfy for the constraint `interface { ~string; IsValid() bool }`?

What must a type satisfy for the constraint `interface { ~string; IsValid() bool }`?

1. It must have underlying type string and an IsValid() bool method.
2. Either a string underlying type or the method is sufficient.
3. Only the predeclared string type can satisfy it.
4. The value is converted to string before the method check.