PR C++/88114 - patch for destructor not generated for "virtual ~destructor() = default"

[Tobias Burnus]

Dear Jason, dear all,

Jason Merrill wrote on 5 Dec 2018:
> You can get at the destructor with CLASSTYPE_DESTRUCTOR rather than walking through TYPE_FIELDS. I'd also check DECL_DEFAULTED_IN_CLASS_P.
> I'd also do this in maybe_emit_vtables rather than here, so that it only happens once per class. 

Updated patch. I also added a test case which checks that the destructor
is not generated.

[ Original patch: https://gcc.gnu.org/ml/gcc-patches/2018-11/msg01824.html ]

Background again:
   If a class contains any 'virtual ... = 0', it's an abstract class and for an
   abstract class, the destructor not added to the vtable.

   For a normal
     virtual ~class() { }
   that's not a problem as the class::~class() destructor will be generated during
   the parsing of the function.

   But for
     virtual ~class() = default;
   the destructor will be generated via mark_used via the vtable.

   If one now declares a derived class and uses it, the class::~class() is generated
   in that translation unit.  Unless, #pragma interface/implementation is used.

   In that case, the 'default' destructor will never be generated.

Build and regtested on x86_64-gnu-linux.
OK? Or do you have more suggested changes?

Tobias
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