[Bug libstdc++/100070] Standard library container iterator-pair constructors should check C++20 iterator concepts

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100070

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Note to self/Patrick:

Measure whether it helps to specialize transform_view's iterator so that when
_Base_iter is __normal_iterator we unwrap it and store a raw pointer.

Also, I suspect the indirections in

  return std::__invoke(*_M_parent->_M_fun, *_M_current);

are making the optimizer give up.

We have an indirection to the parent to access the semi-regular box, which has
its own indirections. Maybe we could just get rid of the semi-regular box for a
function pointer and store a function pointer (i.e. decay_t<Fp>) directly. That
would have the same syntax (i.e. operator*) to access it as the semi-regular
box, but would be less abstraction to un-abstract.

And maybe store a function pointer directly in the transform_view iterator, so
we don't need to go to the parent to get it on every dereference.

Barry pointed out that range-v3 elides the use of semi-regular box for some
cases, and he confirmed that storing a function pointer in the iterator helps.