For example:

struct X {
    X(){/***/};
    X(const int& ){/***/};
    X(      int&&){/***/};
    X(const X& ) = default;
    X(      X&&) = default;
    X& operator=(const X& ){/***/};
    X& operator=(      X&&){/***/};
};

Class X is not trivial and is not an aggregate.

Checking the source code of class X is not an option (although it is known which constructors and operators are available).