This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

2237. Can a template-id name a constructor?

[Accepted at the March, 2018 (Jacksonville) meeting.]

Bullet 1.2 of 11.4.5 [class.ctor], describing declarator forms that are considered to declare a constructor, says:

...and the id-expression has one of the following forms:

• ...

• in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation (13.8.3.2 [temp.dep.type]) of the immediately-enclosing class template; or

• ...

The term class-name includes simple-template-id. It is not clear that allowing a constructor declaration of the form

  template<class T> struct X {
    X<T>(T); // constructor 
  };

is useful or helpful.

Proposed resolution (November, 2017)

1. Change 11.4.5 [class.ctor] paragraph 1 as follows:

...and the id-expression has one of the following forms:

• in a member-declaration that belongs to the member-specification of a class but is not a friend declaration (11.8.4 [class.friend]), the id-expression is the injected-class-name ( Clause 11 [class]) of the immediately-enclosing class;

• in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation (13.8.3.2 [temp.dep.type]) of the immediately-enclosing class template; or

• in a declaration at namespace scope or in a friend declaration, the id-expression is a qualified-id that names a constructor (6.5.5.2 [class.qual]).

2. Change 11.4.7 [class.dtor] paragraph 1 as follows:

...and the id-expression has one of the following forms:

• in a member-declaration that belongs to the member-specification of a class but is not a friend declaration (11.8.4 [class.friend]), the id-expression is ~class-name and the class-name is the injected-class-name (Clause 11 [class]) of the immediately-enclosing class;

• in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is ~class-name and the class-name names the current instantiation (13.8.3.2 [temp.dep.type]) of the immediately-enclosing class template; or

• in a declaration at namespace scope or in a friend declaration, the id-expression is nested-name-specifier ~class-name and the class-name names the same class as the nested-name-specifier.

3. Add the following as a new paragraph in C.3 [diff.cpp17]:

(Note that this resolution is a change for C++20, NOT a defect report against C++17 and earlier versions.)