[clazy] [Bug 430672] New: Level 2 lint copyable-polymorphic is a false positive for pure virtual classes

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https://bugs.kde.org/show_bug.cgi?id=430672

            Bug ID: 430672
           Summary: Level 2 lint copyable-polymorphic is a false positive
                    for pure virtual classes
           Product: clazy
           Version: unspecified
          Platform: unspecified
                OS: Microsoft Windows
            Status: REPORTED
          Severity: minor
          Priority: NOR
         Component: general
          Assignee: unassigned-bugs at kde.org
          Reporter: bepzi at bepzi.com
                CC: smartins at kde.org
  Target Milestone: ---

SUMMARY

The lint for classes that are both copyable and polymorphic triggers for pure
virtual classes, which can't be constructed or copied at all.

The implementation checks if 1) it's polymorphic and 2) if there's a callable
copy constructor or copy-assign constructor:

https://github.com/KDE/clazy/blob/4a4742c750a563180cbf65c2438540cc7e700f7a/src/checks/level2/copyable-polymorphic.cpp#L56-L63

But pure virtual classes can have "callable" (as determined by this code) copy
constructors despite not actually being copyable or even constructable.

STEPS TO REPRODUCE
1. Create a class with a pure virtual method and a not-deleted copy or
copy-assign constructor.
2. Run clazy with the level 2 lint 'copyable-polymorphic' enabled.
3. Observe that clazy flags the pure virtual class for being a copyable
polymorphic.

OBSERVED RESULT

Pure virtual classes are flagged as needing their copy and copy-assign
constructors deleted.

EXPECTED RESULT

The lint should not trigger, because pure virtual classes cannot be constructed
or copied and are not vulnerable to vtable slicing.

SOFTWARE/OS VERSIONS
Windows: 10.0.19042 Build 19042
Qt Version: 5.12.9
Qt Creator: 4.14.0 (Dec 17 2020)
Clazy: (`clazy-standalone.exe --version` only reports that it's based on LLVM
version 11.0.0)

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