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

[Nico Weber via llvm-commits]

"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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