Variadic Templates: Wording for Core Issues 778, 1182, and 1183.

This paper addresses the following three issues with variadic templates:

• 778. Template parameter packs in non-type template parameters.
• 1182. Incorrect description of pack expansion syntax.
• 1183. Expansion of parameter packs in declarators.

Proposed resolution:

1. Change 5.3.3 [expr.sizeof] paragraph 5 as follows:

The identifier in a sizeof... expression shall name a parameter pack. The sizeof... operator yields the number of arguments provided for the parameter pack identifier. The parameter pack is expanded (14.5.3 [temp.variadic]) by the sizeof... operator A sizeof... expression is a pack expansion (14.5.3 [temp.variadic]). [Example:...

2. Change 8.3.5 [dcl.fct] paragraph 13 as follows:

A declarator-id or abstract-declarator containing an ellipsis shall only be used in a parameter-declaration. Such a parameter-declaration is a parameter pack (14.5.3 [temp.variadic]). When it is part of a parameter-declaration-clause, the parameter pack is a function parameter pack (14.5.3 [temp.variadic]). [Note: Otherwise, the parameter-declaration is part of a template-parameter-list and the parameter pack is a template parameter pack; see 14.1 [temp.param]. —end note] The type T of the declarator-id of the function parameter pack shall contain a template parameter pack; each template parameter pack in T is expanded by the function parameter pack A function parameter pack is a pack expansion (14.5.3 [temp.variadic]). [Example:...

3. Change 14.1 [temp.param] paragraph 15 as follows:

If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a parameter pack (8.3.5 [dcl.fct]), then the template-parameter is a template parameter pack (14.5.3 [temp.variadic]). A template parameter pack that is a parameter-declaration whose type contains one or more unexpanded parameter packs is a pack expansion. Similarly, a template parameter pack that is a type-parameter with a template-parameter-list containing one or more unexpanded parameter packs is a pack expansion. A template parameter pack that is a pack expansion shall not expand a parameter pack declared in the same template-parameter-list. [Example:

  template <class ...Types> class Tuple; // Types is a template type parameter pack but not a pack expansion
  template <class T, int ...Dims> struct multi_array; // Dims is a non-type template parameter pack but not a pack expansion
  template<class ...T> struct value_holder {
    template<T ...Values> apply { }; // Values is a non-type template parameter pack and a pack expansion
  };
  template <class ...T, T ...Values> struct static_array; // Error: Values expands template type parameter pack T within the same template parameter list

4. Change 14.5.3 [temp.variadic] paragraphs 4-6 and add a new paragraph 7 as follows:

A pack expansion is a sequence of tokens that names one or more parameter packs, followed by an ellipsis. The sequence of tokens is called the pattern of the expansion; its syntax consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list (described below). The form of the pattern depends on the context in which the expansion occurs. Pack expansions can occur in the following contexts:

• In a function parameter pack (8.3.5 [dcl.fct]); the pattern is the parameter-declaration without the ellipsis.

• In a template parameter pack that is a pack expansion (14.1 [temp.param]):

  • if the template parameter pack is a parameter-declaration; the pattern is the parameter-declaration without the ellipsis,

  • if the template parameter pack is a type-parameter with a template-parameter-list; the pattern is the corresponding type-parameter without the ellipsis.

• ...

• In a sizeof... expression (5.3.3 [expr.sizeof]), the pattern is an identifier.

[Example:...

A parameter pack whose name appears within the pattern of a pack expansion is expanded by that pack expansion. An appearance of the name of a parameter pack is only expanded by the innermost enclosing pack expansion. The pattern of a pack expansion shall name one or more parameter packs that are not expanded by a nested pack expansion; such parameter packs are called unexpanded parameter packs in the pattern. All of the parameter packs expanded...

  ...
  void g(Args ... args) {  // OK: Args is expanded by the function parameter pack args
  ...

The instantiation of an a pack expansion that is not a sizeof... expression produces a list...

The instantiation of a sizeof... expression (5.3.3 [expr.sizeof]) produces an integral constant containing the number of elements in the parameter pack it expands.