Proposed resolution (approved by CWG 2025-11-07):

1. Change in 6.8.2 [intro.object] paragraph 2 as follows:

   Objects can contain other objects, called subobjects. A subobject can be a member subobject (11.4 [class.mem]), a base class subobject (11.7 [class.derived]), or an array element. An object that is not a subobject of any other object is called a complete object. If an object is created in storage associated with a member subobject or array element subobject e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:

   • the lifetime of e's containing object has begun and not ended, and
   • the storage for the new object exactly overlays the storage location associated with e, and
   • e is not a potentially-overlapping subobject, and
   • the new object is of the same type as e (ignoring cv-qualification).

   In this case, e and the created object are corresponding direct subobjects.

2. Change in 6.8.4 [basic.life] paragraph 8 as follows:

   ... An object o1 is transparently replaceable by an object o2 if either

   • o1 and o2 are complete objects for which:
     • o1 is not const,
     • the storage that o2 occupies exactly overlays the storage that o1 occupied, and
     • o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and or
     • o1 is not a const, complete object, and
     • neither o1 nor o2 is a potentially-overlapping subobject (6.8.2 [intro.object]), and
   • either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2 (6.8.2 [intro.object]) for which:
     • the complete object of o1 is not const or
     • o1 is a mutable member subobject or a subobject thereof.

Additional notes (February, 2023)

The suggested resolution above does not handle the additional example in issue 2676.

Suggested resolution [SUPERSEDED]:

Change in 6.8.4 [basic.life] paragraph 8 as follows:

... An object o1 is transparently replaceable by an object o2 if:

• o2 is a complete object and:
  • the storage that o2 occupies exactly overlays the storage that o1 occupied, and
  • o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and
  • o1 is not a const, complete object, and
  • neither o1 nor o2 is not a potentially-overlapping subobject (6.8.2 [intro.object]), and or
• either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2.

[Accepted as a DR at the November, 2025 meeting.]

The resolution to NB comment US 041 (C++20 CD) does not seem to have fully addressed the original issue, allowing:

  union U { int i, j; };
  U u;
  new (&u) U{.i = 5};
  int k = u.j;  // OK! New U::i transparently replaces existing u.j!

The suggestion is to allow a newly-created complete object to transparently replace any object of the same type in the same storage, except for a potentially-overlapping subobject or a const complete object, and to allow a newly-created subobject to transparently replace only a corresponding subobject of an existing object.