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

[Sean Silva]

On Tue, Jan 7, 2014 at 10:26 AM, Argyrios Kyrtzidis <akyrtzi at gmail.com>wrote:

>
> On Jan 7, 2014, at 8:40 AM, Nico Weber <thakis at chromium.org> wrote:
>
> 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).
>
>
> +1
>
> For the ubiquitous assert pattern like:
>
> assert(a || b && “god help us”)
>
> I don’t think this is confusing and the warning offers no value; IMHO
> adding the parentheses is more like noise than improving clarity.
>

FWIW, I recall seeing someone somewhere (can't remember when/where/what
context) say something along the lines of "unlike GCC, Clang is smart
enough to ease off on -Wparentheses when inside the assert macro", which I
take to mean that this is a useful behavior.

-- Sean Silva


>
> Is it out-of-the-question to file a report on GCC to see if they would
> consider also silencing the warning, and in the meantime turn it off for
> gcc ?
>
>
>
>>
>>
>> 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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>>
>
>
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