Proposed resolution (approved by CWG 2026-10-06):

1. Change in 9.1 [dcl.pre] paragraph 11 as follows:

   A nodeclspec-function-declaration shall declare a constructor, destructor, or conversion function. A name-declaration that has the form of a nodeclspec-function-declaration is interpreted as a nodeclspec-function-declaration if and only if its declarator-id has one of the forms specified for the declaration of a constructor, destructor, or conversion function at namespace scope (11.4.5.1 [class.ctor.general], 11.4.7 [class.dtor], 11.4.8.3 [class.conv.fct]).

   [Note 6: Because a member function cannot be subject to a non-defining declaration outside of a class definition (11.4.2 [class.mfct]), a nodeclspec-function-declaration can only be used in a template-declaration (13.1 [temp.pre]), explicit-instantiation (13.9.3 [temp.explicit]), or explicit-specialization (13.9.4 [temp.expl.spec]). —end note]

   [ Example:

     struct A {};
     struct B : A {};
     B::B(A); // error: out-of-line non-defining constructor declaration
     B::A(A); // OK, declares a variable named `A` having type `A`
   

   -- end example]

2. Change in 11.4.5.1 [class.ctor.general] paragraph 1 as follows:

   A declarator declaration declares a constructor if it is a function declarator (9.3.4.6 [dcl.fct]) of

   • in a friend declaration ([class.friend]), the declarator-id is a qualified-id that names a constructor ([class.qual]);
   • otherwise, in a member-declaration that belongs to the member-specification of a class or class template, the declarator-id is the injected-class-name ([class.pre]) of the immediately-enclosing entity;
   • otherwise, the declarator-id is a qualified-id whose unqualified-id is the injected-class-name of its lookup context.

   The declarator of a constructor declaration shall have the form

   ptr-declarator ( parameter-declaration-clause ) noexcept-specifieropt attribute-specifier-seqopt
   

   where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses. , and the id-expression has one of the following forms:

   • in a friend declaration (11.8.4 [class.friend]), the id-expression is a qualified-id that names a constructor (6.5.5.2 [class.qual]);
   • otherwise, in a member-declaration that belongs to the member-specification of a class or class template, the id-expression is the injected-class-name (11.1 [class.pre]) of the immediately-enclosing entity;
   • otherwise, the id-expression is a qualified-id whose unqualified-id is the injected-class-name of its lookup context.

CWG 2026-04-28

See also issue 3160.

Possible resolution (November, 2025) [SUPERSEDED]:

Change in 9.1 [dcl.pre] paragraph 11:

An ambiguity can arise in a declaration when a type-name is enclosed in parentheses. In this case, the choice is between a nodeclspec-function-declaration with an unnamed parameter and a variable declaration with extra parentheses around the declarator-id. The resolution is to consider the declaration a nodeclspec-function-declaration if its declarator-id is a qualified-id and a variable declaration otherwise. A nodeclspec-function-declaration shall declare a constructor, destructor, or conversion function. [Note 6: Because a member function cannot be subject to a non-defining declaration outside of a class definition (11.4.2 [class.mfct]), a nodeclspec-function-declaration can only be used in a template-declaration (13.1 [temp.pre]), explicit-instantiation (13.9.3 [temp.explicit]), or explicit-specialization (13.9.4 [temp.expl.spec]). —end note]

(From submission #741.)

Consider:

  class A;
  A(A);   // #1

#1 can be interpreted as either a nodeclspec-function-declaration or a variable declaration with extra parentheses. A disambiguation rule is missing.