vector lightweight debug mode

[Jonathan Wakely]

On 18/11/15 12:27 +0000, Jonathan Wakely wrote:
>But my unproven assumption is that it's more common to use operator[]
>incorrectly, rather than pass invalid iterators to range insert, which
>is a relatively "advanced" operation.

It's worth noting that the google/integration branch doesn't have any
checks on range insert/erase, and they have entire teams who work on
analysing their code base to find common programming errors, and
identify ways to avoid problems.

That doesn't mean we shouldn't include any checks that aren't in the
google branch, and we also shouldn't use all their checks, but it's
useful to look at what they considered important to check.