[C++11] Why to avoid std::enable_if in function signatures

Why to avoid std::enable_if in function signatures

or simply put:

Use enable_if on template parameter.

• readability:
  the enable_if use and the return/argument types are not merged together into one messy chunk of typename disambiguators and nested type accesses;
  even though the clutter of the disambiguator and nested type can be mitigated with alias templates, that would still merge two unrelated things together. The enable_if use is related to the template parameters not to the return types.
  Having them in the template parameters means they are closer to what matters;
  
• universal applicability:
  constructors don't have return types, and some operators cannot have extra arguments, so neither of the other two options can be applied everywhere.
  Putting enable_if in a template parameter works everywhere since you can only use SFINAE on templates anyway.