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 or class template 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; entity or

• 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 or class template 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; entity or

• 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]:

C.5.x Clause 15: Special member functions [diff.cpp17.special]

Affected subclauses: 11.4.5 [class.ctor], 11.4.7 [class.dtor]
Change: A simple-template-id is no longer valid as the declarator-id of a constructor or destructor.
Rationale: Remove potentially error-prone option for redundancy.
Effect on original feature: Valid C++ 2017 code may fail to compile.

  template<class T>
  struct A {
    A<T>();  // error: simple-template-id not allowed for constructor
    A(int);  // OK, injected-class-name used
    ~A<T>(); // error: simple-template-id not allowed for destructor
  };

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

[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.