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

[Jason Merrill <jason@redhat.com>]

On 11/21/18 7:19 AM, Tobias Burnus wrote:
> Hello all,
> 
> 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.
> 
> 
> The following code seems to work both for the big code and for the example;
> without '#pragma implementation', the destructor is not generated for the example,
> only with.
> 
> The patch survived boostrapping GCC with default languages on x86-64-gnu-linux
> and "make check-g++".*
> 
> [One probably could get rid of some of the conditions for generating the code,
> e.g. TREE_USED and DECL_DEFAULTED_FN are probably not both needed; one might
> want to set some additional DECL to the fn decl.]

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.

Jason