C issue 1016: Are digit separators allowed in __LINE_

Issue 1016: Are digit separators allowed in `LINE`?

Authors: Jens Gustedt
Date: 2025-06-18
Submitted against: C23
Status: Review

Problem description

The introduction of digit separators in C23 seems to have overlooked the case of the __LINE__ macro. The current text would allow them.

If it is possible that such digit separators occur the following code could have expansions that are invalid in compilation phase 7:

#define UNIQ_(L, ...) __VA_ARGS__ ## L
#define UNIQ(...) UNIQ_(__LINE__, __VA_ARGS__)

int UNIQ(special) = 78; callit(UNIQ(special));

In general in all implementations that I am aware of the last line would expand to something like:

int special50138 = 78; callit(special50138);

If the expansion would contain digit separators that would be

int special50'138 = 78; callit(special50'138);

which would be invalid in phase 7.

Proposed solution

Forbid the use of digit separators in the expansion of __LINE__.

Suggested wording

Change the corresponding item in "6.10.10.2 Mandatory macros":

__LINE__ The presumed line number (within the current source file) of the current source line (an integer literal not containing digit separators).²¹⁶⁾

Comment from Issues list maintainer on 2025-09-01:

At the August 2025 (Brno) meeting of WG14, the suggested correction was accepted, subject to review at the next meeting.

An editorial request was made to change "not containing digit separators" to "excluding any digit separators" for consistency with wording already in the standard.

Proposed correction