[PATCH 2/2] color_parse_mem: initialize "struct color" temporary

Compiling color.c with gcc 6.2.0 using -O3 produces some -Wmaybe-uninitialized false positives:

    color.c: In function ‘color_parse_mem’:
    color.c:189:10: warning: ‘bg.blue’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       out += xsnprintf(out, len, "%c8;2;%d;%d;%d", type,
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          c->red, c->green, c->blue);
          ~~~~~~~~~~~~~~~~~~~~~~~~~~
    color.c:208:15: note: ‘bg.blue’ was declared here
      struct color bg = { COLOR_UNSPECIFIED };
                   ^~
    [ditto for bg.green, bg.red, fg.blue, etc]

This is doubly confusing, because the declaration shows it being initialized! Even though we do not explicitly initialize the color components, an incomplete initializer sets the unmentioned members to zero.

What the warning doesn't show is that we later do this:

  struct color c;
  if (!parse_color(&c, ...)) {
          if (fg.type == COLOR_UNSPECIFIED)
                fg = c;
          ...
  }

gcc is clever enough to realize that a struct assignment from an uninitialized variable taints the destination. But unfortunately it's _not_ clever enough to realize that we only look at those members when type is set to COLOR_RGB, in which case they are always initialized.

With -O2, gcc does not look into parse_color() and must assume that "c" emerges fully initialized. With -O3, it inlines parse_color(), and learns just enough to get confused.

We can silence the false positive by initializing the temporary "c". This also future-proofs us against violating the type assumptions (the result would probably still be buggy, but in a deterministic way).

Signed-off-by: Jeff King <peff@peff.net>
---
Of course it's possible that I am wrong and gcc is right, but I just
don't see it (and that was the real reason I dug; I don't care _that_
much about -O3 warnings, but I wanted to see if gcc had found a real
bug).

 color.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)