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

2764. Use of placeholders affecting name mangling

Section: 6.4.1  [basic.scope.scope]     Status: DRWP     Submitter: Hubert Tong     Date: 2023-07-05

[Accepted as a DR at the November, 2023 meeting.]

Paper P2169R4 (A nice placeholder with no name), as approved by WG21 in Varna, added a placeholder facility. The intent was that the use of placeholders is sufficiently limited such that they never need to be mangled. Quote from 6.4.1 [basic.scope.scope] paragraph 5 as modified by the paper:

A declaration is name-independent if its name is _ and it declares a variable with automatic storage duration, a structured binding not inhabiting a namespace scope, the variable introduced by an init-capture, or a non-static data member.

The following example does not seem to follow that intent:

  struct A { A(); };
  inline void f() {
    static union { A _{}; };
    static union { A _{}; };
  }
  void g() { return f(); }

The preceding example needs handling similar to the following example, which is unrelated to the placeholder feature:

  struct A { A(); };
  inline void f() {
    { static union { A a{}; }; }
    { static union { A a{}; }; }
  }
  void g() { return f(); }

A similar problem may arise for static or thread_local structured bindings at block scope.

Finally, another example involving placeholders in anonymous unions:

  static union { int _ = 42; };
  int &ref = _;
  int foo() { return 13; }
  static union { int _ = foo(); };
  int main() { return ref; }

Possible resolution (reviewed by CWG 2023-08-25) [SUPERSEDED]:

Change in 6.4.1 [basic.scope.scope] paragraph 5 and add bullets as follows:

A class is name-dependent if it is an anonymous union declared at namespace scope or with a storage-class-specifier (11.5.2 [class.union.anon]). A declaration is name-independent if its name is _ and it declares

Proposed resolution (approved by CWG 2023-09-15):

Change in 6.4.1 [basic.scope.scope] paragraph 5 and add bullets as follows:

A declaration is name-independent if its name is _ and it declares