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

1014. Overload resolution between const T& and T&&

In the following example,

    template<typename T> void f(const T&);  // #1
    template<typename T> void f(T&&);       // #2
    void g() {
        const int x = 5;
        f(x);
    }

the call f(x) is ambiguous by the current rules. For #1, T is deduced as int, giving

    f<int>(const int&)

For #2, because of the special case for T&& in 13.10.3.2 [temp.deduct.call] paragraph 3, T is deduced as const int&; application of the reference-collapsing rules in 9.3.4.3 [dcl.ref] paragraph 6 to the substituted parameter type yields

    f<const int&>(const int&)

These are indistinguishable in overload resolution, resulting in an ambiguity. It's not clear how this might be addressed.

Rationale (August, 2010):

The two functions are distinguished by partial ordering, so the call is not actually ambiguous.