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

[Stephen Canon]

On Oct 15, 2012, at 12:44 PM, Jordan Rose <jordan_rose at apple.com> wrote:

> 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.

The compiler shouldn't be in the business of suggesting semantically incorrect transformations to the user, especially when there's a perfectly good alternative available.

We don't want to find bugs in users' code only to replace them with even more subtle bugs.

- Steve