C++20 concepts: int not swappable_with int

std::swappable_with<T, U> checks whether swap can be invoked (after using std::swap;) with arguments std::declval<T>() and std::declval<U>(). With T and U being int, both arguments are rvalues, which cannot be bound to std::swap parameters, since these are (non-const) lvalue references.

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You wonder that int cannot be swapped with int — that's right, you cannot write std::swap(1, -1);.

Use std::swappable_with<T&,U&> - swappable with cares about value category, encoded by reference, as well as type.

You are in effect asking if rvalues of type int can be swapped. And it says "no"; you cannot swap to rvalue ints.

This might be confusing, but if you do this:

template <class T, class U>
requires std::swappable_with<T,U>
void mySwap(T&& t, U&& u) {
  auto temp = std::forward<T>(t);
  t = std::forward<U>(u);
  u = std::move(temp);
}

it becomes a bit more natural. Here, we use forwarding references, and the l/rvalue categories of the argument are stored along side the bare types in T and U respectively.

Note that the above could permit rvalues to be swapped, if objects of that type are swappable_with each other.