[Bug lto/78472] New: warning: type of 's' does not match original declaration from zero length bitfield in C vs C++

["jay.foad at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78472

            Bug ID: 78472
           Summary: warning: type of 's' does not match original
                    declaration from zero length bitfield in C vs C++
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jay.foad at gmail dot com
  Target Milestone: ---

If I compile x.h as C and as C++, and link the two together with LTO, I get
this warning, which seems spurious to me:

x.h:5:3: warning: type of 's' does not match original declaration

Here's the repro:

$ cat x.h
extern struct S
{
        unsigned i:4;
        unsigned :0;
} s;
static void *f(void)
{
        return &s;
}

$ gcc -o /dev/null -flto -x c x.h -x c++ x.h
x.h:5:3: warning: type of 's' does not match original declaration
[-Wlto-type-mismatch]
 } s;
   ^
x.h:5:3: note: type 'struct S' should match type 'struct S'
 } s;
   ^
x.h:1:15: note: the incompatible type is defined here
 extern struct S
               ^
x.h:5:3: note: 's' was previously declared here
 } s;
   ^
x.h:5:3: note: code may be misoptimized unless -fno-strict-aliasing is used

$ gcc -v
Using built-in specs.
COLLECT_GCC=/home/jay/gcc/local/bin/gcc
COLLECT_LTO_WRAPPER=/home/jay/gcc/local/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /home/jay/svn/gcc/trunk/configure --prefix=/home/jay/gcc/local
--disable-bootstrap --enable-languages=c,c++,lto
Thread model: posix
gcc version 7.0.0 20161122 (experimental) (GCC) 

I built this compiler from source at svn r242695.