[Bug c++/84097] New: [8 regression] Incorrect -Wunused-but-set-variable warning

[sylvestre at debian dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84097

            Bug ID: 84097
           Summary: [8 regression] Incorrect -Wunused-but-set-variable
                    warning
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sylvestre at debian dot org
                CC: jakub at gcc dot gnu.org, jason at gcc dot gnu.org,
                    sgunderson at bigfoot dot com, trippels at gcc dot gnu.org,
                    unassigned at gcc dot gnu.org, webrown.cpp at gmail dot com
  Target Milestone: ---

+++ This bug was initially created as a clone of Bug #82728 +++

Seen in Firefox code:
/root/firefox-gcc-last/dom/animation/KeyframeUtils.cpp: In member function
'uint32_t
mozilla::PropertyPriorityComparator::SubpropertyCount(nsCSSPropertyID) const':
/root/firefox-gcc-last/layout/style/nsCSSProps.h:689:29: error: variable 'es_'
set but not used [-Werror=unused-but-set-variable]
                             es_ = (nsCSSPropertyID)((enabledstate_) |       \
                             ^~~


#define CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(it_, prop_, enabledstate_)   \
  for (const nsCSSPropertyID *it_ = nsCSSProps::SubpropertyEntryFor(prop_), \
                            es_ = (nsCSSPropertyID)((enabledstate_) |       \
                                                  CSSEnabledState(0));    \
       *it_ != eCSSProperty_UNKNOWN; ++it_)                               \
    if (nsCSSProps::IsEnabled(*it_, (mozilla::CSSEnabledState) es_))


We can see that es_ is used at the last line.

This issue doesn't occur with gcc 7.

Unreduced test case
http://sylvestre.ledru.info/bordel/gcc-unused-but-set-variable.tar.gz 1.1mb
(sorry)

$ g++-8 -c -Werror=unused-but-set-variable foo.cpp