[PATCH] D69626: Fix Microsoft compatibility handling of commas in nested macro expansions.

[Eric Astor via Phabricator via cfe-commits]

epastor added a comment.

In D69626#1728113 <https://reviews.llvm.org/D69626#1728113>, @thakis wrote:

> Is this needed to parse system headers?


Interesting question. This change isn't - but I presume the (pre-existing) introduction of IgnoredComma was.

> In general, we try to emulate only warts we must emulate, and if feasible we try to emit some -Wmicrosoft warning when the compat path is taken.

Sure - this change actually brings the preprocessor closer to compliance. I think the prior use of IgnoredComma had overshot MSVC by a bit. For an illustration, see
https://gcc.godbolt.org/z/XakAYf (same code as above, but in MSVC, clang, and clang -fms-compatibility)
MSVC shows it's non-compliant on TEST, and -fms-compatibility matches it there. However, MSVC matches clang's behavior for WRAPPED_TEST, as adding another layer of macro expansion is a standard trick for forcing MSVC's preprocessor to act compliant... but clang -fms-compatibility still gives the non-compliant result, since that trick doesn't work to remove IgnoredCommas.

If we wanted to detect use of the compatibility feature, we might need to apply the preprocessor both ways and see if the results were different; I'm not sure all IgnoredCommas will actually make a difference in the parsing. As is, we'd need to emit a -Wmicrosoft warning any time we saw an isolated comma as a macro parameter... which I think might get confusing?


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https://reviews.llvm.org/D69626