misbehaves in presence of overloaded operator,

[zygoloid]

Line 116:
template <class X, class Y, size_t S = sizeof((std::declval() = std::declval(), true)) >

... would make use of an overloaded operator, on the result of the assignment. Suggestion: either cast the result of the assignment to void, or use class Z = decltype(... = ...) instead of the third template parameter.