[llvm-bugs] [Bug 43465] New: False positive -Wimplicit-fallthrough warning in flex-generated C code

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43465

            Bug ID: 43465
           Summary: False positive -Wimplicit-fallthrough warning in
                    flex-generated C code
           Product: new-bugs
           Version: trunk
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: sean at rogue-research.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Consider this C code:

-------------------
#include <stdio.h>
#include <stdlib.h>

int main (void)
{
  int x = random();
  switch (x)
  {
    case 1:
      printf("case1");
      /*FALLTHROUGH*/
    default:
      printf("default");
  }
  return 0;
}
-------------------

clang trunk (but not the current release, 9.0) with -Wimplicit-fallthrough
warns:

-------------------
<source>:12:5: warning: unannotated fall-through between switch labels
[-Wimplicit-fallthrough]
    default:
    ^
<source>:12:5: note: insert '__attribute__((fallthrough));' to silence this
warning
    default:
    ^
    __attribute__((fallthrough)); 
<source>:12:5: note: insert 'break;' to avoid fall-through
    default:
    ^
    break; 
-------------------

gcc 7.x and later do *not* warn, because they detect the comment
"/*FALLTHROUGH*/" and take it as a cue that it's deliberately.  This is
documented here:

https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wimplicit-fallthrough

Compare on godbolt:
https://godbolt.org/z/UFpfNm

It would be nice if clang did the same.  Especially since the venerable flex
<https://github.com/westes/flex> generates code that uses exactly that comment.

Without this, clang will start giving false positive warnings (that gcc does
not) on lots of flex-generated code out there.

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