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

2024-12-16

953. Rvalue references and function viability

[Voted into WP at March, 2010 meeting.]

According to 12.2.4.2.5 [over.ics.ref] paragraphs 3-4,

A standard conversion sequence cannot be formed if it requires binding an lvalue reference to non-const to an rvalue (except when binding an implicit object parameter; see the special rules for that case in 12.2.2 [over.match.funcs]). [Note: this means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue reference parameter (other than the implicit object parameter) and the corresponding argument is a temporary or would require one to be created to initialize the lvalue reference (see 9.4.4 [dcl.init.ref]). —end note]

Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of a standard conversion sequence, however.

Because this section does not mention attempting to bind an rvalue reference to an lvalue, such a “conversion sequence” might be selected as best and result in an ill-formed program. It should, instead, be treated like trying to bind an lvalue reference to non-const to an rvalue, making the function non-viable.

Proposed resolution (November, 2009):

Change 12.2.4.2.5 [over.ics.ref] paragraph 3 as follows:

A Except for an implicit object parameter, for which see 12.2.2 [over.match.funcs], a standard conversion sequence cannot be formed if it requires binding an lvalue reference to non-const to an rvalue (except when binding an implicit object parameter; see the special rules for that case in 12.2.2 [over.match.funcs]) or binding an rvalue reference to an lvalue. [Note: this means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue reference parameter (other than the implicit object parameter) and the corresponding argument is a temporary or would require one to be created to initialize the lvalue reference (see 9.4.4 [dcl.init.ref]). —end note]