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Split -Wdelete-non-virtual-dtor into two groups
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Authored by erik.pilkington on Mon, Jan 7, 12:15 PM.

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Summary

-Wdelete-non-virtual-dtor controlled two diagnostics: 1) calling a non-virtual dtor from an abstract class, and 2) calling a non-virtual dtor from a polymorphic class. 1) is a lot more severe than 2), since 1) is a guaranteed crash, but 2) is just "code smell". Previously, projects compiled with -Wall -Wno-delete-non-virtual-dtor, which is somewhat reasonable, silently crashed on 1).

rdar://40380564

Thanks for taking a look!

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rC Clang

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This revision is now accepted and ready to land.Mon, Jan 7, 2:00 PM
This revision was automatically updated to reflect the committed changes.
rsmith added inline comments.Mon, Jan 7, 4:51 PM
include/clang/Basic/DiagnosticGroups.td
108–109

This is backwards: this says that -Wdelete-abstract-non-virtual-dtor also controls -Wdelete-non-virtual-dtor. You presumably want the opposite relationship, so that -Wdelete-non-virtual-dtor controls both warnings and -Wdelete-abstract-non-virtual-dtor only controls the "abstract" warning.

test/SemaCXX/non-virtual-dtors.cpp
3–4 ↗(On Diff #180584)

The test also verifies that the -W flags are wrong :)

aaron.ballman added inline comments.Mon, Jan 7, 4:59 PM
include/clang/Basic/DiagnosticGroups.td
108–109

I took this to be the correct order because disabling the abstract case is more dangerous than disabling the non-abstract case (if you disable the abstract one, you're saying "I don't care how bad it gets, don't tell me about it.").

rsmith added inline comments.Mon, Jan 7, 5:56 PM
include/clang/Basic/DiagnosticGroups.td
108–109

That seems reasonable as a strategy, but the end result doesn't seem to make much sense: -Wdelete-abstract-non-virtual-dtor enables, and -Wno-delete-abstract-non-virtual-dtor disables, warnings that have nothing to do with deleting an abstract class with a non-virtual destructor, and -Wno-delete-non-virtual-dtor fails to silence warnings about deleting an object of a class type with a non-virtual destructor. It's also backwards-incompatible, because the meaning of the existing -W flag has been changed.

One way to fix this would be to rename the groups:

  • delete-abstract-non-virtual-dtor -> delete-non-virtual-dtor
  • delete-non-virtual-dtor -> delete-nonabstract-non-virtual-dtor (yuck)

(Or we could keep the existing delete-abstract-non-virtual-dtor, add delete-nonabstract-non-virtual-dtor, and make delete-non-virtual-dtor be a group that contains those other two groups and has no diagnostics of its own.)

Instead / as well, we could address the false positives more directly: we could only warn if the class in question *introduces* a virtual function (suggesting that it's intended to be used as a base class), rather than warning if the class merely *has* virtual functions (if it overrides virtual functions and doesn't introduce any, there's a good chance it's a leaf class). -Wdelete-non-virtual-dtor was supposed to be the "few/no false positives" version of -Wnon-virtual-dtor (which is really really just a stylistic warning), and if we can improve it so that people don't want to turn it off, that'd seem better.

aaron.ballman added inline comments.Tue, Jan 8, 6:32 AM
include/clang/Basic/DiagnosticGroups.td
108–109

That seems reasonable as a strategy, but the end result doesn't seem to make much sense: -Wdelete-abstract-non-virtual-dtor enables, and -Wno-delete-abstract-non-virtual-dtor disables, warnings that have nothing to do with deleting an abstract class with a non-virtual destructor, and -Wno-delete-non-virtual-dtor fails to silence warnings about deleting an object of a class type with a non-virtual destructor. It's also backwards-incompatible, because the meaning of the existing -W flag has been changed.

Ah, those are all good points!

(Or we could keep the existing delete-abstract-non-virtual-dtor, add delete-nonabstract-non-virtual-dtor, and make delete-non-virtual-dtor be a group that contains those other two groups and has no diagnostics of its own.)

I have a slight preference for this approach; it feels a bit more natural to me. However, do we want to spell it delete-nonabstract-non-virtual-dtor or delete-non-abstract-non-virtual-dtor or delete-nonabstract-nonvirtual-dtor? My preference is for anything but the first spelling. ;-)

Instead / as well, we could address the false positives more directly:

Yes, improving the fp rate that way would be a great change to make. That said, I would view it as "as well" rather than "instead" because these two diagnostic scenarios seem reasonably separable.

erik.pilkington reopened this revision.Tue, Jan 8, 9:37 AM
erik.pilkington marked an inline comment as done.

I reverted my commit in r350639.

include/clang/Basic/DiagnosticGroups.td
108–109

Instead / as well, we could address the false positives more directly: we could only warn if the class in question *introduces* a virtual function (suggesting that it's intended to be used as a base class), rather than warning if the class merely *has* virtual functions (if it overrides virtual functions and doesn't introduce any, there's a good chance it's a leaf class). -Wdelete-non-virtual-dtor was supposed to be the "few/no false positives" version of -Wnon-virtual-dtor (which is really really just a stylistic warning), and if we can improve it so that people don't want to turn it off, that'd seem better.

I think the (vast?) majority of users follow the rule that every polymorphic class has a virtual dtor (or, at the very least, would never delete a polymorphic class without one). It seems like they would want the diagnostic even if the class didn't introduce any more virtual functions. So I don't want to weaken the "not a guaranteed crash" part of -Wdelete-non-virtual-dtor with this heuristic. We might be able to put it in the "guaranteed crash" half, but that might lead to people disabling it on bad codebases, and I don't think anyone should ever disable the "guaranteed crash" diagnostic. A third option would be to have three flags controlling this, but it doesn't seem like that's worth the complexity.

(Or we could keep the existing delete-abstract-non-virtual-dtor, add delete-nonabstract-non-virtual-dtor, and make delete-non-virtual-dtor be a group that contains those other two groups and has no diagnostics of its own.)

I have a slight preference for this approach; it feels a bit more natural to me. However, do we want to spell it delete-nonabstract-non-virtual-dtor or delete-non-abstract-non-virtual-dtor or delete-nonabstract-nonvirtual-dtor? My preference is for anything but the first spelling. ;-)

Ya, I think this is probably best. It does have the downside that existing "-Wall -Wno-delete-non-virtual-dtor" builds will still silently accept the crasher, but I guess we can't make that do the right thing and still have sane cli semantics.

This revision is now accepted and ready to land.Tue, Jan 8, 9:37 AM
erik.pilkington planned changes to this revision.Tue, Jan 8, 9:37 AM

Split -Wdelete-non-virtual-dtor into -Wdelete-non-abstract-non-virtual-dtor and -Wdelete-abstract-non-virtual-dtor.

This revision is now accepted and ready to land.Tue, Jan 8, 10:06 AM
erik.pilkington requested review of this revision.Wed, Jan 9, 12:36 PM
This revision is now accepted and ready to land.Thu, Jan 10, 8:03 AM
This revision was automatically updated to reflect the committed changes.