[Bug libstdc++/92300] Useless allocator call in std::map, when insert does not perform any insertion.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92300

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-10-31
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
make_pair(1, 2) creates a std::pair<int, int> which is not the same type as the
value_type. That means there's a conversion from pair<int, int> to pair<const
int, int>, which gets done by allocating a node, constructing the value_type
into the node, then checking if the key exists. That's the expected behaviour
when inserting something that is not the same type as the value_type, so I
don't think this is a bug.

We could special-case pair<key_type, mapped_type> to avoid that step, because
we can use the non-const key directly. Converting it to a const key isn't
needed to check if it exists.

In fact we could special-case pair<cv key_type, anything>.

Confirming as an enhancement request.