Improve insert/emplace robustness to self insertion

[Jonathan Wakely]

On 29/06/16 11:35 +0200, Paolo Carlini wrote:
>Hi,
>
>On 29/06/2016 10:57, Jonathan Wakely wrote:
>>On 28/06/16 21:59 +0200, François Dumont wrote:
>>>+      if (_M_data_ptr(__position.base()) <= __ptr
>>>+      && __ptr < _M_data_ptr(this->_M_impl._M_finish - 1))
>>
>>This is undefined behaviour. If the object is not contained in the
>>vector then you can't compare its address to addresses within the
>>vector.
>
>Uhm, would that be true also if the code used std::less? Aren't we 
>doing something like that in std::basic_string under the assumption 
>(Nathan?) that it would not be the case? Or maybe I'm misreading the 
>code (admittedly I didn't follow in detail the whole exchange)

Yes, it's OK if you use std::less. In general there's no guarantee
that std::less<T*> defines the same order as operator<(T*, T*), but in
our implementation it does.

I'd still prefer to keep a correctness fix and a potentially risky
optimisation in separate commits.