gcc "missing initializer" warning with stmt expr

The combination of union with anonymous struct member and initializer with statement expression containing a declaration causes a spurious "missing initializer for field" warning.

I am using arm-none-eabi-gcc 5.4.1 20160919. I am using the project-compiled binaries. The bug is also present in 4.9.3 20141119 and 5.2.1 20151202.

Host machine is MacOS 10.11.6.

Transcript:

~$ cat > init-bug.c
union gfx_point {
    struct {
        float x, y;
    };
};

union gfx_point line_p0;

void animate(void)
{
    line_p0 = (union gfx_point) {
#ifdef NO_ERROR
        .x = ({ 0; }),
#else
        .x = ({
            float z = 0;
            z;
        }),
#endif
        .y = 0,
    };
}
~$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 5.4.1 20160919 (release) [ARM/embedded-5-branch revision 240496]
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

~$ arm-none-eabi-gcc -Wextra -c -std=c11 init-bug.c
init-bug.c: In function 'animate':
init-bug.c:20:9: warning: missing initializer for field 'y' of 'struct <anonymous>' [-Wmissing-field-initializers]
         .y = 0,
         ^
init-bug.c:3:18: note: 'y' declared here
         float x, y;
                  ^
~$ arm-none-eabi-gcc -Wextra -c -std=c11 init-bug.c -DNO_ERROR
~$