Put the hack in the template parameters.
The enable_if
on template parameter approach has at least two advantages over the others:
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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;
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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.
For me, the readability aspect is the big motivating factor in this choice.