This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2026-05-08

3179. More edge cases for a void function parameter

(From submission #889.)

Consider:

  template<class T>
  struct S {
    using U = decltype((void)(T*)0);
    void f(U);   // #1
  };
  template<class T>
  void S<T>::f() {}  // redeclaration of #1?

Also consider:

  template<class T>
  struct S {
    void f(std::void_t<T*>);  // #1
  };
  S<int> x;  // #1 is a zero-parameter function or has an ill-formed parameter of type void

The current phrasing in 9.3.4.6 [dcl.fct] paragraph 2 is the result of core issues 577 and 2915. The current rule, enabling typedefs for void, is primarily motivated by C compatibility.

Possible resolution (2026-04-22) [SUPERSEDED]:

Change in 9.3.4.6 [dcl.fct] paragraph 3 as follows and split into two paragraphs:

The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [Note 1: The parameter-declaration-clause is used to convert the arguments specified on the function call; see 7.6.1.3 [expr.call]. —end note] If the parameter-declaration-clause is empty, the function takes no arguments. A parameter list (void) and, for a non-templated function, a parameter list consisting of a single unnamed non-object parameter of non-dependent type void is are equivalent to an empty parameter list. Except for this these special case cases, a parameter shall not have type cv void. A parameter with volatile-qualified type is deprecated; see D.4 [depr.volatile.type].

If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack (13.7.4 [temp.variadic]), the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where “...” is not part of an abstract-declarator , “...” is synonymous with “, ...”. A parameter-declaration-clause of the form parameter-declaration-list ... is deprecated (D.5 [depr.ellipsis.comma]). ...

CWG 2026-04-28

Despite the fact that "equivalent type" is not well-defined (see issue 2584), the most appropriate approach is to use "equivalent type". Both examples above are ill-formed.

Proposed resolution (approved by CWG 2026-05-08):

Change in 9.3.4.6 [dcl.fct] paragraph 3 as follows and split into two paragraphs:

The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [Note 1: The parameter-declaration-clause is used to convert the arguments specified on the function call; see 7.6.1.3 [expr.call]. —end note] If the parameter-declaration-clause is empty, the function takes no arguments. A parameter list consisting of a single unnamed non-object parameter of non-dependent some type T, where T is equivalent to (13.7.7.2 [temp.over.link]) void, is equivalent to an empty parameter list. Except for this special case, a parameter shall not have type cv void. A parameter with volatile-qualified type is deprecated; see D.4 [depr.volatile.type].

[ Example:

  template<class T>
  struct S {
    void f(std::void_t<T*>);  // #1
  };
  S<int> x;  // error: #1 has a parameter of type void

  template<class T>
  struct S2 {
    void f(std::void_t<int*>);  // #2
  };
  S2<int> y;  // OK, #2 is a zero-parameter function

-- end example ]

If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack (13.7.4 [temp.variadic]), the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where “...” is not part of an abstract-declarator , “...” is synonymous with “, ...”. A parameter-declaration-clause of the form parameter-declaration-list ... is deprecated (D.5 [depr.ellipsis.comma]). ...