std::declval is actually:
template<class T>
typename std::add_rvalue_reference<T>::type declval() noexcept;
Where std::add_rvalue_reference<T> is usually T&&, except in cases where that is invalid (Like if T = void or T = int() const), where it is just T. The main difference is that functions cannot return arrays, but can return array references like U(&&)[] or U(&&)[N].
The problem with explicitly using std::add_rvalue_reference is that it instantiates a template. And that itself instantiates around 10s of templates at an instantiation depth of ~4 in the libstdc++ implementation. In generic code, std::declval can be used a lot, and according to https://llvm.org/bugs/show_bug.cgi?id=27798, there is a >4% compile time boost by not using std::add_rvalue_reference. (The libc++ implementation instantiates less templates, but it still has an impact)
This is fixed by inlining the “add_rvalue_reference” directly into declval. This is done using SFINAE.
The return type for declval<T> is decltype(__declval<_Tp>(0)). When looking up __declval, two function templates are found.
The first has return type _Up = T&&. The second just has return type T.
The first takes a parameter int, and the second long. It is being passed 0, which is an int, so the first function is a better match and is chosen, and T&& is returned.
Except, when T&& is not a valid type (e.g., T = void), then when the template argument _Up is substituted with the deduced T&&, there is a substitution failure. So it is no longer a candidate for the function. That means only the second one is left, and the 0 is converted into a long (And the return type is just T).
In cases where T and T&& can’t be returned from a function (e.g., T = int() const), neither function can be picked, and the std::declval<T> function has a substitution failure and is not a viable candidate.
Here is the libc++ commit introducing the optimisation: https://github.com/llvm/llvm-project/commit/ae7619a8a358667ea6ade5050512d0a27c03f432
And here is the libstdc++ commit: https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=ec26ff5a012428ed864b679c7c171e2e7d917f76
They were both previously std::add_rvalue_reference<T>::type