This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1700. Does the special rvalue-reference deduction apply to alias templates?

An rvalue reference type involving a template parameter receives special treatment in template argument deduction in 13.10.3.2 [temp.deduct.call] paragraph 3:

If P is an rvalue reference to a cv-unqualified template parameter and the argument is an lvalue, the type “lvalue reference to A” is used in place of A for type deduction.

Does this rule apply when the parameter type involves an alias template instead of using a template parameter directly? For example:

    template<class T> using RR = T&&;

    template<class T> struct X {};
    template<class T> struct X<T&&>; // Leave incomplete to possibly trigger an error

    template<class T> void g(RR<T> p) {
      X<decltype(p)> x;
    }

    int main() {
      int x = 2;
      g(x);
    }

There is implementation variance on the treatment of this example.

Additional note (October, 2013):

It was observed that the type of the parameter would be the same whether written as T&& or as RR<T>, which would require that deduction be performed the same, regardless of how the type was written.

Rationale (September, 2013):

Because the types of the function parameters are the same, regardless of whether written directly or via an alias template, deduction must be handled the same way in both cases.