https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100070
--- 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 = allocator_type()) : _Base(__a) { +#if __cpp_lib_concepts + if constexpr (forward_iterator<_InputIterator>) + { + size_type __n; + if constexpr (random_access_iterator<_InputIterator>) + __n = __last - __first; + else + __n = 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 __last, std::forward_iterator_tag) { - const size_type __n = 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 = this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator())); this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n; This doubles the speed of the vector construction, but it's still slow because it uses std::copy to copy the range elements into the vector, and std::copy also dispatches on the iterator_category.