[Bug c/68656] New: [4.8, 4.9, 5.0, 6.0 Regression] warning about disabled var-tracking uses odd location info

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68656

            Bug ID: 68656
           Summary: [4.8, 4.9, 5.0, 6.0 Regression] warning about disabled
                    var-tracking uses odd location info
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aldot at gcc dot gnu.org
                CC: dodji at gcc dot gnu.org
  Target Milestone: ---

echo "/* This is a bugreport about using odd locations.  */" > bug.c
gcc -c bug.c -fvar-tracking -o /dev/null 
bug.c:1:0: warning: variable tracking requested, but useless unless producing
debug info
 /* This is a bugreport about using odd locations.  */
 ^

on stdin this looks like expected:
$ echo "/* This is a bugreport about using odd locations.  */" | gcc -xc -c -
-g -fvar-tracking -gtoggle -o /dev/null
<stdin>:1:0: warning: variable tracking requested, but useless unless producing
debug info
$ 

Known to work: 4.7.4
$ gcc-4.7 -c bug.c -fvar-tracking -o /dev/null 
bug.c:1:0: warning: variable tracking requested, but useless unless producing
debug info [enabled by default]

Fails: 4.8.5
$ gcc-4.8 -c bug.c -fvar-tracking -o /dev/null 
bug.c:1:0: warning: variable tracking requested, but useless unless producing
debug info [enabled by default]
 /* This is a bugreport about using odd locations.  */
 ^