Proposed resolution (approved by CWG 2024-02-16):

Change in 9.3.4.6 [dcl.fct] paragraph 6 as follows:

An explicit-object-parameter-declaration is a parameter-declaration with a this specifier. An explicit-object-parameter-declaration shall appear only as the first parameter-declaration of a parameter-declaration-list of either one of:

• a member-declarator that declares declaration of a member function or member function template (11.4 [class.mem]), or
• an explicit instantiation (13.9.3 [temp.explicit]) or explicit specialization (13.9.4 [temp.expl.spec]) of a templated member function, or
• a lambda-declarator (7.5.5 [expr.prim.lambda]).

A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared static or virtual.

[Accepted as a DR at the March, 2024 meeting.]

(From submission #493.)

Consider:

  struct A {
    void f(this A&);
  };
  void A::f(this A&) { }    // #1

This is accepted by all major implementations. However, 9.3.4.6 [dcl.fct] paragraph 6 specifies:

An explicit-object-parameter-declaration is a parameter-declaration with a this specifier. An explicit-object-parameter-declaration shall appear only as the first parameter-declaration of a parameter-declaration-list of either:

• a member-declarator that declares a member function (11.4 [class.mem]), or
• a lambda-declarator (7.5.5 [expr.prim.lambda]).

A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared static or virtual.

The function-definition at #1 is neither of the two allowed options.

Similar concerns arise for explicit instantiations and explicit specializations.