[Bug bootstrap/77974] New: m68k bootstrap failure due to -Werror=implicit-fallthrough

["mikpelinux at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77974

            Bug ID: 77974
           Summary: m68k bootstrap failure due to
                    -Werror=implicit-fallthrough
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: bootstrap
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mikpelinux at gmail dot com
  Target Milestone: ---

Attempting to bootstrap gcc-7-20161009 on m68k-linux fails with:

/mnt/scratch/objdir7/./prev-gcc/xg++ -B/mnt/scratch/objdir7/./prev-gcc/
-B/mnt/scratch/install7/m68k-unknown-linux-gnu/bin/ -nostdinc++
-B/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/src/.libs
-B/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs

-I/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/include/m68k-unknown-linux-gnu
 -I/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/include 
-I/mnt/scratch/gcc-7-20161009/libstdc++-v3/libsupc++
-L/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/src/.libs
-L/mnt/scratch/objdir7/prev-m68k-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs
-fno-PIE -c   -g -O2 -gtoggle -DIN_GCC     -fno-exceptions -fno-rtti
-fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings
-Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic
-Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror  
-DHAVE_CONFIG_H -I. -I. -I/mnt/scratch/gcc-7-20161009/gcc
-I/mnt/scratch/gcc-7-20161009/gcc/.
-I/mnt/scratch/gcc-7-20161009/gcc/../include
-I/mnt/scratch/gcc-7-20161009/gcc/../libcpp/include 
-I/mnt/scratch/gcc-7-20161009/gcc/../libdecnumber
-I/mnt/scratch/gcc-7-20161009/gcc/../libdecnumber/dpd -I../libdecnumber
-I/mnt/scratch/gcc-7-20161009/gcc/../libbacktrace   -o m68k.o -MT m68k.o -MMD
-MP -MF ./.deps/m68k.TPo /mnt/scratch/gcc-7-20161009/gcc/config/m68k/m68k.c
/mnt/scratch/gcc-7-20161009/gcc/config/m68k/m68k.c: In function 'const char*
m68k_get_reloc_decoration(m68k_reloc)':
/mnt/scratch/gcc-7-20161009/gcc/config/m68k/m68k.c:4537:4: error: this
statement may fall through [-Werror=implicit-fallthrough]
    if (TARGET_68020)
    ^~
/mnt/scratch/gcc-7-20161009/gcc/config/m68k/m68k.c:4551:5: note: here
     case RELOC_TLSGD:
     ^~~~
cc1plus: all warnings being treated as errors
make[3]: *** [m68k.o] Error 1
make[3]: Leaving directory `/mnt/scratch/objdir7/gcc'
make[2]: *** [all-stage2-gcc] Error 2
make[2]: Leaving directory `/mnt/scratch/objdir7'
make[1]: *** [stage2-bubble] Error 2
make[1]: Leaving directory `/mnt/scratch/objdir7'
make: *** [bootstrap] Error 2

I believe gcc-7-20161002 had the same issue.  gcc-7-20160918 bootstrapped fine.

Configuration options:
/mnt/scratch/gcc-7-20161009/configure --prefix=/mnt/scratch/install7
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++
--disable-sjlj-exceptions --disable-libmudflap --disable-plugin --disable-lto
--disable-multilib --disable-libgomp