关于std::is_swappable_with_v判断结果与实际swap调用不一致的技术问询

Great question—this inconsistency between compiler behavior is definitely confusing, especially since cppreference explicitly states that a valid swap call (after using std::swap;) should correspond to is_swappable_with<T,U>::value being true.

Let’s break down what’s happening here:

1. Standard Definition of std::is_swappable_with #

Per the C++ standard, std::is_swappable_with<T, U> evaluates to true if and only if:

• The expressions swap(t, u) and swap(u, t) are well-formed, where t is an lvalue of type T and u is an lvalue of type U.
• The lookup for swap in this context includes std::swap (via an implicit using std::swap; equivalent) plus any overloads found via Argument-Dependent Lookup (ADL) for the types T and U.

2. Why Your swap Call is Valid #

Your global swap template:

template <class T, class U> constexpr void swap(A<T>&, A<U>&) {}

is a perfect match for swap(A<int>&, A<double>&) (and vice versa). When you call swap(a,b) with lvalues a and b, ADL finds this template in the global namespace (since A<T> is defined there), so all compilers accept it without issue.

3. Why GCC/Clang Reject the static_assert #

This appears to be an implementation bug in GCC and Clang. Their is_swappable_with checks are failing to properly resolve the ADL lookup for your templated swap function when verifying cross-type A<T>/A<U> swaps.

The critical point here is cppreference’s explicit guarantee: if the actual swap call compiles successfully, is_swappable_with_v should return true. MSVC’s acceptance of the static_assert aligns with this guarantee, while GCC/Clang’s rejection does not.

You can confirm this is a template-specific bug by testing a non-templated swap overload (e.g., void swap(A<int>&, A<double>&)): GCC/Clang will correctly accept the static_assert in that case, proving the issue lies in how their is_swappable_with implementation handles templated cross-type swap overloads.

Conclusion #

Yes, GCC and Clang are incorrect to reject the static_assert here. This deviates from the standard’s requirements and cppreference’s documented behavior. You can report this as a bug to the respective compiler teams if you wish.

内容的提问来源于stack exchange，提问作者463035818_is_not_an_ai