[C++ Patch / RFC] Change DERIVED_FROM_P to use tf_none?!?

[Paolo Carlini]

On 07/22/2013 09:22 PM, Jason Merrill wrote:
> OK.
Thanks, applied.
>> (*) Note that, depending on how it's called, the predicate can 
>> currently produce errors anyway, for example because it calls 
>> complete_type. This may or may not be all there is to c++/57942.
> It looks like lookup_base seems to deliberately avoid trying to 
> complete the base, so the call to complete_type is coming from elsewhere.
I see, indeed the comment in lookup_base:

   /* If BASE is incomplete, it can't be a base of T--and instantiating it
      might cause an error.  */

is very clear. Now, I tell you briefly what is going on: 
standard_conversion calls ptr_reasonably_similar, which, in turn calls 
comptypes. The latter, via structural_comptypes, does:

       if ((strict & COMPARE_BASE) && DERIVED_FROM_P (t1, t2))
     break;
       else if ((strict & COMPARE_DERIVED) && DERIVED_FROM_P (t2, t1))
     break;

you see, DERIVED_FROM_P, thus lookup_base, handles *both* the pair 
(base, derived) and the swapped pair (derived, base), thus for sure in 
one case the above comment / code doesn't help, because it protects vs 
the instantiation of the base argument not vs the instantiation of the t 
argument... bummer. I guess fixing the issue must be rather doable but 
at the moment I'm not clear about where to act...

Thanks!
Paolo.