[Bug libc/27885] New: _Static_assert fallback definition in <sys/cdefs.h> triggers a lgtm.com alert

["gjover at sipwise dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27885

            Bug ID: 27885
           Summary: _Static_assert fallback definition in <sys/cdefs.h>
                    triggers a lgtm.com alert
           Product: glibc
           Version: 2.34
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: gjover at sipwise dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The fallback definition for _Static_assert in <sys/cdefs.h>, which is currently
(as of git HEAD glibc-2.33.9000-613-g4d4bb451d5f):

  #if (!defined _Static_assert && !defined __cplusplus \
       && (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) < 201112 \
       && (!__GNUC_PREREQ (4, 6) || defined __STRICT_ANSI__))
  # define _Static_assert(expr, diagnostic) \
      extern int (*__Static_assert_function (void)) \
        [!!sizeof (struct { int __error_if_negative: (expr) ? 2 : -1; })]
  #endif

causes the lgtm.com analyzer to trigger, on code using that keyword, the
following alert:

  https://lgtm.com/rules/1506024027114/

which is described as:

  Ambiguously signed bit-field member

  Bit fields with integral types should have explicit signedness only. For 
  example, use `unsigned int` rather than `int`. It is implementation specific 
  whether an `int`-typed bit field is signed, so there could be unexpected sign 
  extension or overflow.

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