[PATCH] D44883: [Sema] Extend -Wself-assign and -Wself-assign-field to warn on overloaded self-assignment (classes)

[Roman Lebedev via cfe-commits]

Because *so far* it has been running on the existing code, which i guess
was already pretty warning free, and was run through the static analyzer,
which obviously should catch such issues.

Do you always use [clang] static analyzer, each time you build?
Great! It is indeed very good to do so. But does everyone?

This particular issue is easily detectable without full static analysis,
and as it is seen from the differential, was very straight-forward to implement
as a simple extension of pre-existing -Wself-assign.

So if this is really truly unacceptable false-positive rate, ok, sure,
file a RC bug,
and i will split it so that it can be simply disabled for *tests*.

That being said, i understand the reasons behind "keep clang diagnostics
false-positive free". I don't really want to change that.

On Tue, Apr 10, 2018 at 7:13 PM, David Blaikie <dblaikie at gmail.com> wrote:
> It's more the fact that that's /all/ the warning improvement has found so
> far. If it was 8 false positives & also found 80 actionable bugs/bad code,
> that'd be one thing.
>
> Now, admittedly, maybe it would help find bugs that people usually catch
> with a unit test, etc but never make it to checked in code - that's always
> harder to evaluate though Google has some infrastructure for it at least.
>
> On Tue, Apr 10, 2018 at 9:07 AM John McCall <rjmccall at gmail.com> wrote:
>>
>> If you have a concrete suggestion of how to suppress this warning for
>> user-defined operators just in unit tests, great. I don’t think 8
>> easily-suppressed warnings is an unacceptably large false-positive problem,
>> though. Most warnings have similar problems, and the standard cannot
>> possibly be “must never fire on code where the programmer is actually happy
>> with the behavior”.
>>
>> John.
>> On Tue, Apr 10, 2018 at 17:12 Nico Weber <thakis at chromium.org> wrote:
>>>
>>> "False positive" means "warning fires but didn't find anything
>>> interesting", not "warning fires while being technically correct". So all
>>> these instances do count as false positives.
>>>
>>> clang tries super hard to make sure that every time a warning fires, it
>>> is useful for a dev to fix it. If you build with warnings enabled, that
>>> should be a rewarding experience. Often, this means dialing back a warning
>>> to not warn in cases where it would make sense in theory when in practice
>>> the warning doesn't find much compared to the amount of noise it generates.
>>> This is why for example clang's -Woverloaded-virtual is usable while gcc's
>>> isn't (or wasn't last I checked a while ago) – gcc fires always when it's
>>> technically correct to do so, clang only when it actually matters in
>>> practice.
>>>
>>> On Tue, Apr 10, 2018 at 10:21 AM, Roman Lebedev via Phabricator via
>>> cfe-commits <cfe-commits at lists.llvm.org> wrote:
>>>>
>>>> lebedev.ri added a comment.
>>>>
>>>> In https://reviews.llvm.org/D44883#1063003, @thakis wrote:
>>>>
>>>> > This landing made our clang trunk bots do an evaluation of this
>>>> > warning :-P It fired 8 times, all false positives, and all from unit tests
>>>> > testing that operator= works for self-assignment.
>>>> > (https://chromium-review.googlesource.com/c/chromium/src/+/1000856 has the
>>>> > exact details) It looks like the same issue exists in LLVM itself too,
>>>> > https://reviews.llvm.org/D45082
>>>>
>>>>
>>>> Right, i guess i only built the chrome binary itself, not the tests,
>>>> good to know...
>>>>
>>>> > Now tests often need warning suppressions for things like this, and
>>>> > this in itself doesn't seem horrible. However, this change takes a warning
>>>> > that was previously 100% noise-free in practice and makes it pretty noisy –
>>>> > without a big benefit in practice. I get that it's beneficial in theory, but
>>>> > that's true of many warnings.
>>>> >
>>>> > Based on how this warning does in practice, I think it might be better
>>>> > for the static analyzer, which has a lower bar for false positives.
>>>>
>>>> Noisy in the sense that it correctly diagnoses a self-assignment where
>>>> one **intended** to have self-assignment.
>>>> And unsurprisingly, it happened in the unit-tests, as was expected ^ in
>>>> previous comments.
>>>> **So far** there are no truly false-positives noise (at least no reports
>>>> of it).
>>>>
>>>> We could help workaround that the way i initially suggested, by keeping
>>>> this new part of the diag under it's own sub-flag,
>>>> and suggest to disable it for tests. But yes, that
>>>>
>>>>
>>>>
>>>> Repository:
>>>>   rC Clang
>>>>
>>>> https://reviews.llvm.org/D44883
>>>>
>>>>
>>>>
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