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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100070] Standard library container iterator-pair
 constructors should check C++20 iterator concepts
Date: Tue, 13 Apr 2021 22:11:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100070

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a partial solution:

--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -654,6 +654,19 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
               const allocator_type& __a =3D allocator_type())
        : _Base(__a)
        {
+#if __cpp_lib_concepts
+         if constexpr (forward_iterator<_InputIterator>)
+           {
+             size_type __n;
+             if constexpr (random_access_iterator<_InputIterator>)
+               __n =3D __last - __first;
+             else
+               __n =3D std::distance(__first, __last);
+             _M_range_initialize_n(__first, __last, __n);
+             return;
+           }
+         else
+#endif
          _M_range_initialize(__first, __last,
                              std::__iterator_category(__first));
        }
@@ -1577,7 +1590,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
        _M_range_initialize(_ForwardIterator __first, _ForwardIterator __la=
st,
                            std::forward_iterator_tag)
        {
-         const size_type __n =3D std::distance(__first, __last);
+         _M_range_initialize_n(__first, __last,
+                               std::distance(__first, __last));
+       }
+
+      template<typename _InputIterator>
+       void
+       _M_range_initialize_n(_InputIterator __first, _InputIterator __last,
+                             size_type __n)
+       {
          this->_M_impl._M_start
            =3D this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocato=
r()));
          this->_M_impl._M_end_of_storage =3D this->_M_impl._M_start + __n;


This doubles the speed of the vector construction, but it's still slow beca=
use
it uses std::copy to copy the range elements into the vector, and std::copy
also dispatches on the iterator_category.=