Summary

SFINAE is a somewhat esoteric feature of the C++ standard - it is complex and has many subtle details. While it is usually mentioned in the context of manual control of the overload resolution, its main purpose is actually not to enable very elaborate guru-level code but to make the regular (automatic) overload resolution work the way the programmer intended. In this role, it usually works exactly as desired and with no additional effort - in fact, the programmer usually does not need to even be aware of this feature. Most of the time, when you write a generic overload and a special overload for the pointers, you expect the latter not to be called for types that are not pointers. Most of the time, you probably don’t even pause to notice that the rejected overload would be ill-formed - who cares, it’s not supposed to be used. But to find out that it’s not supposed to be used, the type has to be substituted, which would result in an invalid code. SFINAE breaks...