http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58687
--- Comment #22 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Fri, 29 Nov 2013, mtewoodbury at gmail dot com wrote: > The elaborate description of the different forms of the '#line' (and other) > directives makes it clear that <pp-token> expansion is not to take place > until > after the <end-of-line> for the directive has been seen. The question is not when it takes place, it's what the "current token" is when it takes place, because "line number" is defined in terms of the current token rather than the time of expansion. I think the current token is clearly __LINE__ on the #line directive. Consider the case where there are no more tokens after that directive, just the newline at the end of the directive (you can easily construct cases where the value of __LINE__ in such a context affects the validity of the program, by concatenating it with something else to form what might or might not be a macro name) - it's not possible the the current token to be anything on a subsequent line, because there aren't any subsequent tokens. So the current token has got to be __LINE__ (or maybe, in the more complicated cases I referred to, something else on the #line line). That's different from macro ( __LINE__ ) where all of the tokens from "macro" to ")" are involved in expansion at the same time and you can argue for any of "macro", "__LINE__" and ")" as being the "current token". (It's also different from the cases where backslash-newline appears in the middle of __LINE__ so the question is whether "to the current token" means to the start of end of that token.) > Accepted usage is for '#line __LINE__' to leave the line numbering unchanged. > It's quite possible there's a common bug among multiple other implementations where it does leave the line number unchanged, but I'm now tending to that actually being a bug in those implementations rather than an ambiguity in the standard. That is, there is no way in the standard to leave the line number unchanged, and no defect, and it might be better to wait until a revision process for a new major C standard version is underway them submit a proposal for e.g. #line with just a filename and no line number (or a #file directive - either would be a reasonable way of achieving that goal).
