Issue 1016: Are digit separators allowed in __LINE__?

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

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⁾