[PATCH] RFC: fail-fast iterators for DenseMap

[Sanjoy Das]

On Mon, Mar 2, 2015 at 8:05 PM, Chandler Carruth <chandlerc at google.com> wrote:
> (relaying from IRC)
>
> The key is that default constructing an iterator must form a valid end
> iterator for an empty range. This is a requirement for a lot of iterator
> types, and so the epoch stuff should support it. I think it would be fine to
> say that the epoch test is passed if both handles have a nullptr epoch
> address or both handles have a non-null epoch address and the epoch's match
> for both. Seem reasonable?

Yes.

>
> On Mon, Mar 2, 2015 at 7:49 PM, Sanjoy Das <sanjoy at playingwithpointers.com>
> wrote:
>>
>> As far as I can tell, DeclContext::lookups [1] tries to create an
>> empty range using two default constructed
>> DeclContext::all_lookups_iterator[2].
>> DeclContext::all_lookups_iterator contains DenseMapIterators.
>>
>> [1]:
>> https://github.com/llvm-mirror/clang/blob/master/include/clang/AST/DeclLookups.h#L81
>> [2]:
>> https://github.com/llvm-mirror/clang/blob/master/include/clang/AST/DeclLookups.h#L35
>>
>> On Mon, Mar 2, 2015 at 7:21 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>> >
>> >
>> > On Tue, Mar 3, 2015 at 12:53 PM, Sanjoy Das
>> > <sanjoy at playingwithpointers.com>
>> > wrote:
>> >>
>> >> I'm about to revert this because it broke clang.
>> >>
>> >> The reason it broke clang is that clang assumes a default constructed
>> >> DenseMapIterator is comparable with another default constructed
>> >> DenseMapIterator.
>> >
>> >
>> >
>> > Where does this happen?
>> > I'd really like to see an actual example of how this makes sense to do
>> >
>> > i have no idea whether it's generally considered "normal" to do this,
>> > but
>> > i'm actually really curious to see a case where it is the right thing to
>> > do
>> > :)
>> >
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>
>