Edit: noticed you're using these in another place (`compatible` implementation), so perhaps there's a reason for a different approach?
Ideally you'd want to do enable_if< conditionA || conditionB >, but of course if one of the conditions fails to evaluate, the overload is ignored. So you have to split out the conditions and them merge them back together later on.
We could use true_type / false_type, but they have equivalent sizes. So unlike the function version that only needs one test and can just take the return type directly, the test at the end would then have to become std::is_same<decltype(test1<U>(0)), std::true_type>::value | std::is_same<decltype(test2<U>(0)), std::true_type>::value.
I still think we can do better than even this, so I'll have to keep working at it.
I highly suspect that C++ is unreasonably complicated and that all of this rvalue-reference, variadic template, meta-programming, dynamic-casting polymorphism, is all just voodoo that isn't applicable to general programming. And yet, being able to do it gives me amazing expressive power to write awesome libraries that I could never hope to accomplish in another language.
Until I find a language that's even in the same ballpark as C++ in terms of performance, and offers similar expressiveness, it really doesn't even matter how bad C++ can be for library authors. There's no other viable option right now. D is the closest we have, but its complexity already rivals, if not exceeds, that of C++.