C++ trait for "has no constructors" OR trait for "is zero-initializable"?

cppreference says for zero initialization:

Zero initialization is performed in the following situations: ... 2) As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, ...

What trait or combination of traits can I use to test either for:

• type is zero-initializable with an empty {}, or
• class type has "no constructors"
  • (which I think means no user-defined constructors?)

(P.S. tagged C++11 to indicate any "modern C++" approach is acceptable; also tagged C++17 to indicate the latest C++ version I'd like for an answer.)

(P.P.S. I know cppreference is not the standard, but it is almost always correct. If it is not correct in this case, that should be part of the answer ...)