Patch for PR 9399

Tom Tromey
Thu Dec 10 19:04:00 GMT 2009

>>>>> "Daniel" == Daniel Jacobowitz <> writes:

Daniel> Tom's approved this.  That's fine.

I'm open to unapproving it if you think it is incorrect.
I don't want to paper over a bug.

>> value_cast_structs only does nothing if both TYPE_NAME()s are null.
>> I was wondering if, back when the code was originally written, if
>> there never was a case when both TYPE_NAME()s were non-null--it's the
>> only way, other than simple oversight, I can account for that case
>> not being covered.

Daniel> No, it looks exactly like I intended.  If we have two classes, we
Daniel> check if A is a base class of B, then if B is a base class of A.
Daniel> If A == B, neither of those checks will be true, and we'll return
Daniel> without doing anything.

Daniel> So the problem is that one of those base class checks is calling
Daniel> error().  That's not supposed to happen.

I think from value_cast_structs:

  /* Downcasting: look in the type of the target to see if it contains the
     type of the source as a superclass.  If so, we'll need to
     offset the pointer rather than just change its type.
     FIXME: This fails silently with virtual inheritance.  */

... but this doesn't fail silently, it tries to dereference NULL, due to
the use of value_zero.  IIRC.

The reason I approved this is that it seems strange to me to try to do
any cast when we can tell beforehand that it is not needed.  It is doing
a lot of work for no reason.

Also, I tried a little to write a different test that would still hit
this failure, and I couldn't.  I don't know if Chris tried this or not.


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