This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

1164. Partial ordering of f(T&) and f(T&&)

[Voted into the WP at the November, 2010 meeting.]

The following example is ambiguous:

    template<typename T> int f(T&);
    template<typename T> int f(T&&);
    int i;
    int j = f(i);

Because of the special deduction rule for lvalues passed to rvalue-reference parameters, deduction produces f(int&) for both templates, and they are indistinguishable.

Because f(T&) accepts a strict subset of the things that f(T&&) does, it should be considered more specialized by the partial ordering rules.

Proposed resolution (August, 2010):

Change 13.10.3.5 [temp.deduct.partial] paragraph 9 as follows:

If, for a given type, deduction succeeds in both directions (i.e., the types are identical after the transformations above) and both P and A were reference types (before being replaced with the type referred to above):

• If the type from the argument template was an lvalue reference and the type from the parameter template was not, the argument type is considered to be more specialized than the other; otherwise,

• and if the type from the argument template is more cv-qualified than the type from the parameter template (as described above), the argument that type is considered to be more specialized than the other.; otherwise

• If neither type is more cv-qualified than the other then neither type is more specialized than the other.

[Editing note: this change transforms the running text at the end of the paragraph into a bulleted list.]