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

527. Problems with linkage of types

[Voted into WP at October, 2009 meeting.]

The resolution of issue 389 makes code like

    static struct {
        int i;
        int j;
    } X;

ill-formed. This breaks a lot of code for no apparent reason, since the name X is not known outside the translation unit in which it appears; there is therefore no danger of collision and no need to mangle its name.

There has also been recent discussion on the email reflectors as to whether the restrictions preventing use of types without linkage as template arguments is needed or not, with the suggestion that a mechanism like that used to give members of the unnamed namespace unique names could be used for unnamed and local types. See also issue 488, which would become moot if types without linkage could be used as template parameters.

Notes from the October, 2005 meeting:

The Evolution Working Group is discussing changes that would address this issue. CWG will defer consideration until the outcome of the EWG discussions is clear.

Notes from the April, 2006 meeting:

The CWG agreed that the restriction in 6.6 [basic.link] paragraph 8 on use of a type without linkage should apply only to variables and functions with external linkage, not to variables and functions with internal linkage (i.e., the example should be accepted). This is a separate issue from the question before the EWG and should be resolved independently.

Additional note (April, 2006):

Even the restriction of the rule to functions and objects with external linkage may not be exactly what we want. Consider an example like:

    namespace {
        struct { int i; } s;
    }

The variable s has external linkage but can't be named outside its translation unit, so there's again no reason to prohibit use of a type without linkage in its declaration.

Notes from the June, 2008 meeting:

Paper N2657, adopted at the June, 2008 meeting, allows local and unnamed types to be used as template parameters. That resolution is narrowly focused, however, and does not address this issue.

Proposed resolution (June, 2009):

Change 6.6 [basic.link] paragraph 8 as follows:

...A type without linkage shall not be used as the type of a variable or function with external linkage, unless

• the variable or function has extern "C" C language linkage (9.11 [dcl.link]), or

• the variable or function is declared within an unnamed namespace (9.8.2 [namespace.def]), or

• the variable or function is not used (6.3 [basic.def.odr]) or is defined in the same translation unit.

[Drafting note: the context shown for the preceding resolution assumes that the resolution for issue 757 has been applied.]