Problem building Emacs b1fe27d with clang/LLVM 10.x

[Ashish SHUKLA]

Hi,

I am one of the maintainers of editors/emacs-devel FreeBSD port. Future
releases of FreeBSD (12-STABLE, or -CURRENT) will have LLVM 10.x as the
base compiler toolchain. We received reports[1][2] of Emacs build
failing with them:

Following is an excerpt from the build log:

=========================================================

gmake[2]: Entering directory
'/wrkdirs/usr/ports/editors/emacs-devel/work-full/emacs-b1fe27d/lib-src'
cc   -Wno-switch -Wno-pointer-sign -Wno-string-plus-int
-Wno-unknown-attributes -Wno-initializer-overrides
-Wno-tautological-compare
-Wno-tautological-constant-out-of-range-compare  -I. -I../src -I../lib
-I. -I./../src -I./../lib  -fstack-protector-strong -L/usr/local/lib
-L/usr/local/lib -isystem /usr/local/include -O2 -pipe
-fstack-protector-strong -isystem /usr/local/include
-fno-strict-aliasing  -o etags etags.c  ../lib/libgnu.a -lutil
error: fallthrough annotation does not directly precede switch label
makeinfo --force -I./../emacs --no-split  \
  -o ../../info/bovine.info bovine.texi
1 error generated.
gmake[2]: *** [Makefile:366: etags] Error 1
gmake[2]: Leaving directory
'/wrkdirs/usr/ports/editors/emacs-devel/work-full/emacs-b1fe27d/lib-src'
gmake[1]: *** [Makefile:411: lib-src] Error 2
/bin/mkdir -p ./info
gmake[1]: *** Waiting for unfinished jobs....

=========================================================

As a workaround for this problem I have locally implemented following
diff which seem to have fixed the issue:

--- lib-src/etags.c.orig        2020-05-18 16:17:29 UTC
+++ lib-src/etags.c
@@ -4196,7 +4196,6 @@ C_entries (int c_ext, FILE *inf)
              objdef = omethodsign;
              break;
            }
-         FALLTHROUGH;
        resetfvdef:
        case '#': case '~': case '&': case '%': case '/':
        case '|': case '^': case '!': case '.': case '?':


My C/C++-fu is very outdated. I'm wondering if any of the Emacs
developers have any better fixes for the problem. If you like I can file
a bug report for the same.

Also, as I'm not subscribed to the list, if you could please Cc me, and
address@hidden in your replies.

References:
[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246525
[2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246668

Thanks!
-- 
Ashish SHUKLA

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