[Bug regression/78497] New: compiling with -save-temps adds -Wimplicit-fallthrough warnings

["aoliva at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78497

            Bug ID: 78497
           Summary: compiling with -save-temps adds -Wimplicit-fallthrough
                    warnings
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aoliva at gcc dot gnu.org
  Target Milestone: ---

Since -save-temps performs separate preprocessing and drops comments, and
-Wimplicit-fallthrough machinery recognizes some of the fallthrough comments to
silence warnings, we get additional warnings (or errors, with -Werror) when
compiling with -save-temps vs without it.

It occurred to me that the preprocessor must be looking out for such comments
in order for -Wimplicit-fallthrough to work, and in case of separate
preprocessing, it would be desirable to retain information about the internal
state implied by the removed comments in a #line directive, so that even with
separate preprocessing we'd still have the same state to work with during
compilation.

This may require introducing artificial line breaks and blanks in the
preprocessed output, so as to introduce the state at the right point in the (to
be reconstructed) token stream.