[PATCH] RFC: fail-fast iterators for DenseMap

[Duncan P. N. Exon Smith]

> On 2015 Mar 2, at 20:37, Chandler Carruth <chandlerc at google.com> wrote:
> 
> 
> On Mon, Mar 2, 2015 at 8:35 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> > For `==` and `!=, you shouldn't worry about whether the handle is in sync
> > with the debug base -- if anything, you should just check that the two
> > handles are pointing at the same debug base (and have the same epoch as
> > each other).  (But I still don't see how you've modified `==` or `!=`.)
> >
> > The comparison operators call operator->
> 
> (Sorry, should have just looked at the code myself.)
> 
> That's silly though.  They should just be:
> 
>     bool operator==(const ConstIterator &RHS) const { return Ptr == RHS.Ptr; }
>     bool operator!=(const ConstIterator &RHS) const { return Ptr != RHS.Ptr; }
> 
> Calling `->` is needlessly complicated.
> 
> Sure.
> 
> But I think we *should* check the epoch here as comparing an invalid iterator with a valid iterator should also be caught. I would check that the address of the epoch are the same, and if they are non-null, that all three epoch's (the pointed too and both iterator's copies) are in sync.

SGTM.