[Bug tree-optimization/87926] New: bad array-index warning breaks --disable-checking bootstrap

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87926

            Bug ID: 87926
           Summary: bad array-index warning breaks --disable-checking
                    bootstrap
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 44967
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44967&action=edit
cc1plus bug.ii -O2 -W -Wall -quiet

x86_64-linux --disable-checking bootstrap fails with:
../../../src/gcc/bitmap.c: In function ‘unsigned int
bitmap_last_set_bit(const_bitmap)’:
../../../src/gcc/bitmap.c:1191:26: error: array subscript -1 is below array
bounds of ‘const BITMAP_WORD [2]’ {aka ‘const long unsigned int [2]’}
[-Werror=array-bounds]
  1191 |       word = elt->bits[ix];
       |              ~~~~~~~~~~~~^

Attached is a reduced testcase

Adding:
bitmap.o-warn = -Wno-error
to gcc/Makefile.in works around the problem.