[Bug c/88660] New: Invalid report of "set but used variable" with -O

[alexander.hirsch at gin dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88660

            Bug ID: 88660
           Summary: Invalid report of "set but used variable" with -O
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alexander.hirsch at gin dot de
  Target Milestone: ---

Created attachment 45314
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45314&action=edit
testcase triggering the warning

Compiling the attached testcase with -O -Wunused-but-set-variable on gcc >= 8.1
erroneously generates a warning that i is unused.
Removing either the -O flag, the const-modifier or replacing the
switch-statement by e.g. an if leads to no warning being reported.
The warning also does not appear in gcc 7.4.