This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 121a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2026-10-16

3234. explicit and eligible constructors

Consider:

  template <class T>
  struct X {
    X() = default;
    X(const X&) = default;
  private:
    explicit X(const X&) requires (T::value) = default;  // #1
  };

  struct S { static constexpr bool value = true; };

  int main() {
    const X<S> x;
    X<S> x2 = x;     // #2
  }

According to 11.4.4 [special] paragraph 6, the public copy constructor is not eligible, because #1 is more constrained. This causes it to be deleted (9.6.2 [dcl.fct.def.default] paragraph 5). Yet, the public copy constructor is selected by overload resolution when used at #2.

Proposed resolution (approved by CWG 2026-10-16):

Change in 11.4.4 [special] paragraph 6 as follows:

An eligible special member function is a special member function for which:

• the function is not deleted,
• the associated constraints (13.5 [temp.constr]), if any, are satisfied, and
• there exists no special member function
  • that is of the same kind,
  • that is not more explicit,
  • whose associated constraints, if any, are satisfied, and
  • that is more constrained (13.5.5 [temp.constr.order]),
  where a function F1 is more explicit than a function F2 if F1 is explicit (9.2.3 [dcl.fct.spec]) and F2 is not.

[ Example:

  template <class T>
  struct X {
    X() = default;
    X(const X&) = default;                         // #1
  private:
    explicit X(const X&) requires true = default;  // #2
  };

  int main() {
    X<int> x;
    X<int> x2 = x;     // OK, selects #1; both #1 and #2 are eligible
  }

-- end example]

CWG 2026-10-06

See also GCC bugzilla #127627.