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4622. Is NULL allowed to be expanded to an extension?

Section: 17.2.3 [support.types.nullptr] Status: New Submitter: Jiang An Opened: 2026-09-14 Last modified: 2026-09-19

Priority: Not Prioritized

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Discussion:

In some current implementations (e.g., in GCC and Clang's own <stddef.h> header), the NULL macro may be expanded to the __null extension (demo). Although __null can be used interchangeably with zero-value integer literals of the same type in most contexts, its exact spelling is observable.

LWG 4182⁽ⁱ⁾ requires NULL to be expanded to a literal, and a pedantic reading may suggest that only standard literals are permitted. In other words, if a program requires NULL to be expanded as a standard literal (in either compile time or runtime), LWG 4182⁽ⁱ⁾ seemingly requires implementations to support it, which in turns renders the existing use of the __null extension non-conforming.

Proposed resolution: