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

2106. Unclear restrictions on use of function-type template arguments

[Adopted at the February, 2016 meeting.]

According to 13.4.2 [temp.arg.type] paragraph 3,

If a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed.

This is not clear enough regarding which declarations are in view. For example, does it apply to a typedef declaration? Does it apply to a parameter declaration, where normal function-to-pointer decay would apply? There is implementation variance at block scope.

Also, since this applies a restriction to the usage of dependent types, not template type arguments per se, the paragraph presumably should appear in 13.8.3.2 [temp.dep.type] and not its current location.

Proposed resolution (September, 2015):

1. Delete 13.4.2 [temp.arg.type] paragraph 3:

If a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed. [Example:

  template<class T> struct A {
    static T t;
  };
  typedef int function();
  A<function> a; // ill-formed: would declare A<function>::t
                 // as a static member function

—end example]

2. Add the following as a new paragraph following 13.9 [temp.spec] paragraph 6:

...X<int> has a static member s of type int and X<char*> has a static member s of type char*. —end example]

If a function declaration acquired its function type through a dependent type (13.8.3.2 [temp.dep.type]) without using the syntactic form of a function declaator, the program is ill-formed. [Example:

   template<class T> struct A {
     static T t;
   };
   typedef int function();
   A<function> a;   // ill-formed: would declare A<function>::t
                    // as a static member function

—end example]