[cfe-commits] [Patch] Add new warning for logical not on LHS of comparison

[Jordan Rose]

On Oct 14, 2012, at 8:19 , Stephen Canon <scanon at apple.com> wrote:

> On Oct 12, 2012, at 12:38 PM, Richard Trieu <rtrieu at google.com> wrote:
> 
>> This patch is for a new warning to Clang, called -Wlogical-not-compare.  It is designed to catch the case where the user is attempting to negate a comparison, but only manages to negate the LHS because of missing parens.  For instance, warn here:
>> 
>>     if (!x < 5)
>> 
>> The user probably meant:
>> 
>>     if (!(x < 5))
>> 
>> or
>> 
>>     if (x >= 5)
>> 
>> When emitted, the warning will have a note suggesting this as a fix-it (drop the not and inverting the comparison operator).
> 
> If x has FP type, then !(x < 5) is not equivalent to (x >= 5); if x is NaN, the first is true, but the second is false.

The fixit is on a note, so I personally think it's okay that it doesn't perfectly match the semantics of the existing expression. I think most people ignore NaNs when writing casual floating-point code.

Jordan