This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

4620. Clarify whether there is any requirement for "the behavior is unspecified (possibly ill-formed)"

Section: 16.4.5.2.1 [namespace.std] Status: New Submitter: Jiang An Opened: 2026-09-09 Last modified: 2026-09-19

Priority: Not Prioritized

View other active issues in [namespace.std].

View all other issues in [namespace.std].

View all issues with New status.

Discussion:

Since C++20/P0551R3, in 16.4.5.2.1 [namespace.std], we say some constructs (taking address of a non-addressable function etc.) may cause the behavior of a program unspecified and possibly ill-formed. This sounds very similar to "ill-formed, no diagnostic required", but it is unclear whether there are still some imposed requirements so we should not say IFNDR.

Perhaps it is better to either

• clarify that there are still some requirements imposed for these cases so it is unsuitable to say IFNDR, or

• rephrase these restrictions to say IFNDR.

Proposed resolution: