[Bug debug/104564] New: '-fcompare-debug' failure w/ -std=c++20 -O1 -fharden-conditional-branches -fopenacc -gno-statement-frontiers

["asolokha at gmx dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104564

            Bug ID: 104564
           Summary: '-fcompare-debug' failure w/ -std=c++20 -O1
                    -fharden-conditional-branches -fopenacc
                    -gno-statement-frontiers
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: compare-debug-failure
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: asolokha at gmx dot com
  Target Milestone: ---
            Target: aarch64-linux-gnu

gcc 12.0.1 20220213 snapshot (g:58aeb75d4097010ad9bb72b964265b18ab284f93) fails
-fcompare-debug check when compiling the following testcase, partially reduced
from libstdc++-v3/testsuite/25_algorithms/inplace_merge/1.cc, w/ -std=c++20 -O1
-fharden-conditional-branches -fopenacc -gno-statement-frontiers:

#include <algorithm>
#include <testsuite_iterators.h>

typedef
  __gnu_test::test_container<int, __gnu_test::bidirectional_iterator_wrapper>
  container;

void test1() {
  int array[] = {};
  container con2(array, array);
  std::inplace_merge(con2.begin(), con2.begin(), con2.end());
}

% aarch64-linux-gnu-g++-12.0.1 -std=c++20 -O1 -fcompare-debug
-fharden-conditional-branches -fopenacc -gno-statement-frontiers -c bdxhor92.cc
aarch64-linux-gnu-g++-12.0.1: error: bdxhor92.cc: '-fcompare-debug' failure

It is very fragile, even adding -save-temps makes the failure disappear, so I
could not reduce it any further. If I get the dumps right, the difference
starts in einline.