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

1662. Capturing function parameter packs

[Moved to DR at the February, 2014 meeting.]

The example in 7.5.6 [expr.prim.lambda] paragraph 24 reads,

  template<class... Args>
  void f(Args... args) {
    auto lm = [&, args...] { return g(args...); };
    lm();
  }

However, it's not clear how this example squares with the requirements in paragraph 10,

The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup (6.5.3 [basic.lookup.unqual]); each such lookup shall find a variable with automatic storage duration declared in the reaching scope of the local lambda expression.

since a function parameter pack is not a variable. Although the requirement might be met in a given specialization of the function template, is there a rule that relaxes it in the context of the function template definition?

Proposed resolution (September, 2013):

1. Change 7.5.6 [expr.prim.lambda] paragraph 10 as follows:

The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup (6.5.3 [basic.lookup.unqual]); each such lookup shall find an entity. An entity that is designated by a simple-capture is said to be explicitly captured, and shall be this or a variable with automatic storage duration declared in the reaching scope of the local lambda expression. An entity (i.e. a variable or this) is said to be explicitly captured if it is found by this process.

2. Change 13.7.4 [temp.variadic] paragraph 4 as follows:

...Pack expansions can occur in the following contexts:

• ...

For the purpose of determining whether a parameter pack satisfies a rule regarding entities other than parameter packs, the parameter pack is considered to be the entity that would result from an instantiation of the pattern in which it appears.

[Example:

...

3. Change 13.7.4 [temp.variadic] paragraph 6 as follows:

...Each E_i is generated by instantiating the pattern and replacing each pack expansion parameter with its ith element. Such an element, in the context of the instantiation, is interpreted as follows:

• if the pack is a template parameter pack, the element is a template parameter (13.2 [temp.param]) of the corresponding kind (type or non-type) designating the type or value from the template argument; otherwise,

• if the pack is a function parameter pack, the element is an id-expression designating the function parameter that resulted from the instantiation of the pattern where the pack is declared.

All of the E_i become elements in the enclosing list. [Note:...