This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2837. Instantiating and inheriting by-value copy constructors

(From submission #474.)

Subclause 11.4.5.3 [class.copy.ctor] paragraph 5 specifies:

A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv X and either there are no other parameters or else all other parameters have default arguments. A member function template is never instantiated to produce such a constructor signature.

It is unclear what happens when such a constructor signature is inherited from a base class. Also, the wording "never instantiated" is not detailed enough.

Suggested resolution:

1. Add a paragraph before 9.9 [namespace.udecl] paragraph 13 as follows:

   The set of declarations named by a using-declarator that inhabits a class C does not include any constructor whose first parameter has type cv C and all of whose remaining parameters (if any) have default arguments.

   Constructors that are named by a using-declaration are treated as though they were constructors of the derived class ...

2. Change in 11.4.5.3 [class.copy.ctor] paragraph 5 as follows:

   A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv X and either there are no other parameters or else all other parameters have default arguments. A member function template is never instantiated to produce such a constructor signature. During type deduction for a constructor template of X (13.10.3.1 [temp.deduct.general]), if substitution produces such a constructor signature, type deduction fails. [ Example 5:

     struct S {
       template<typename T> S(T);
       S();
     };
     S g;
     void h() {
       S a(g);  // does not instantiate the member template to produce S::S<S>(S); no candidate generated from the member template;
                // uses the implicitly declared copy constructor is used
     }
   

   -- end example]