[C++ Patch] PR 90909 ("[10 Regression] call devirtualized to pure virtual")

[Jason Merrill]

On 6/20/19 12:24 AM, Paolo Carlini wrote:
> Hi,
> 
> this bug notices that the more aggressive de-virtualization check that 
> we have now in place (fixed c++/67184) doesn't work correctly for the 
> below reproducer, which involves a pure virtual: we de-virtualize and 
> the build fails at link-time. To cure this I believe we simply want an 
> additional DECL_PURE_VIRTUAL_P in the condition.

I don't see why this bug would be specific to pure virtual functions, so 
the fix also should not be.  If S1::f is not pure virtual, I'd expect 
that we wrongly devirtualize to it in the same way.

Devirtualizing the call in S4 is a good optimization, we're just 
selecting the wrong function.

It seems like the issue here is that the using-declaration hides the 
final overrider from lookup.  So we need to work harder to find the 
actual final overrider, perhaps by looking into the argtype vtable.

Jason