[cfe-dev] Strange difference between Clang and GCC's -Wparentheses

Nico Weber thakis at chromium.org
Tue Jan 7 08:40:01 PST 2014

No opinion on the actual issue, but…

On Mon, Jan 6, 2014 at 11:49 PM, Chandler Carruth <chandlerc at gmail.com>wrote:

> Currently Clang and GCC warn about:
> (a || b && c)
> Due to the very common mistake by programmers which ignores the relative
> operator precedence of || and &&. This forces the user to place explicit
> parentheses to group either (a || b) or (b && c) to be evaluated first.
> However, GCC warns and Clang is silent about this:
> (a || b && 1)
> Worse, the same is true for:
> (a || b && "foo")
> Now, I find Clang's rationale seems somewhat reasonable: the grouping
> doesn't change the truth table for this expression. However, I find this
> clever interpretation problematic for several reasons:
> 1) It's a complex rule to teach programmers with dubious gains. This
> exception doesn't realistically make the warning's false positives rare, it
> just eliminates one common (but not the only common) pattern. On the flip
> side, it is not a trivial rule to teach or predict for programmers. *Any*
> constant that we can evaluate to true will satisfy this.
> 2) If the programmer expected the || to be evaluated first, they might
> well also expect the "a" expression to be evaluated first and short
> circuiting to take place. Even though the truth table is unimpacted,
> 'assert(!ptr || ptr->empty() && "foo");' can segfault, which I think might
> surprise users.
> 3) It makes it annoying to track warnings in both Clang and GCC. If you
> mostly develop with Clang, but someone else checks out your code with GCC
> it'll suddenly start warning (much like it just did in a recent commit.)

…this isn't a convincing argument. Several of clang's warnings are by
design smarter and less noisy than gcc's, with the effect that the clang
warning is useful while the gcc counterpart isn't
(e.g. Woverloaded-virtual, but there are many more).

> Thoughts? I'm happy to fix this, the change is trivial in SemaExpr.cpp,
> but wanted to get some confirmation from Richard and Argyrios who have
> worked on this warning that such a change would be OK.
> -Chandler
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> cfe-dev at cs.uiuc.edu
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