[Bug c++/77886] -Wimplicit-fallthrough: breaks duff's device (in function templates)

[marc.mutz at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77886

--- Comment #7 from Marc Mutz <marc.mutz at kdab dot com> ---
Version:

  $ g++ -v
  Using built-in specs.
  COLLECT_GCC=g++
 
COLLECT_LTO_WRAPPER=/d/gcc/trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
  Target: x86_64-pc-linux-gnu
  Configured with: ../gcc/configure --prefix=/d/gcc/trunk
--enable-checking=release --enable-languges=c,c++,fortran,lto,objc,obj-c++
  Thread model: posix
  gcc version 7.0.0 20161005 (experimental) (GCC) 

Specifically:

  $ git log -10 --online
  c7b01e7         PR sanitizer/77823      * c-ubsan.c (ubsan_instrument_shift):
Return NULL_TREE if type0         is not integral.
  d204444 Fix pr69941.c test failure for avr
  ea55eab 2016-10-05  Jerry DeLisle  <jvdelisle@gcc.gnu.org>
  9ce1157 PR bootstrap/77819 - undefined reference to
gnu_libc_printf_pointer_format with uClibc
  5c176eb         * c-common.c (c_common_reswords): Update comment for C++11.
  4ef2832 [fold-const] Fix native_encode_real for HFmode constants
  59deb1a 70564 fix newly-added tests for not_fn
  c375ef0 libcpp/ChangeLog:
  77a1a89 Move all existing strchr and strrchr folding from builtins.c to
gimple-fold.c.
  ad69f5a PR 70101 fix allocator-extended ctors for std::priority_queue