On Wed, Sep 14, 2016 at 12:07:49AM -0400, Jason Merrill wrote:
> On Sat, Sep 10, 2016 at 10:58 AM, Marek Polacek <pola...@redhat.com> wrote:
> > Spurred by the recent <http://www.viva64.com/en/b/0425/> findings, I
> > decided to
> > implement a warning that warns when a pointer is compared with a zero
> > character
> > literal (constant), because this isn't likely to be intended. So e.g.
> >
> > ptr == L'\0'
> >
> > is probably wrong and should've been written as
> >
> > ptr[0] == L'\0'
> >
> > Jonathan pointed out that this would actually be invalid C++11 since pointer
> > conversions are only allowed for integer literals, not char literals.
>
> Ah, indeed. And if we fix that, we get an error rather than a
> warning. Maybe let's handle this by wrapping character literals in a
> redundant NOP_EXPR?
So I've tried. Wrapping character literals in a NOP_EXPR would make us
error on that comparison (good), but it breaks -Wchar-subscripts, which
could be solved by adding STRIP_NOPS, but unfortunately it breaks even
-Wmemset-transposed-args: '\0' would become (char) '\0', which is not a
literal zero anymore. And if I do sth like
+ if (TREE_CODE (arg2) == NOP_EXPR
+ && TREE_TYPE (arg2) == TREE_TYPE (TREE_OPERAND (arg2, 0)))
+ arg2 = TREE_OPERAND (arg2, 0);
then (int) 0 would became a literal zero and we'd warn when not appropriate.
We should also error for e.g. void *p = '\0'; but the problem with the
NOP_EXPR cast is that convert_for_init strips nops, so we wouldn't reject
this code.
So it seems we may need some CHARACTER_CST or somesuch?
Note that we should also take boolean-literals into account.
Marek
