Our parallel algorithms currently expect for the begin/end iterator pairs to be of the same type. This is an unnecessary limitation. We should relax that.

Here is the list of algorithms that need to be adapted:

• adjacent_difference (Adjacent #5564)
• adjacent_find (Adapt adjacent_find to C++20 #4996)
• all_of any_of none_of
• copy copy_if (Implement customization point for hpx::copy and hpx::ranges::copy #4821)
• move
• count count_if
• equal (Adapting hpx::equal to be conforming to C++20 #4883) mismatch (Adapting mismatch to C++20 #4884)
• exclusive_scan (Adapt exclusive_scan to C++20 #5436)
• inclusive_scan (Adapt inclusive_scan to C++ 20 #5439)
• reduce (see Allowing for end iterator being different from begin iterator #3650, Adding CPOs for hpx::reduce #4836)
• transform (Adapting transform to C++20 (non-compilable PR) #4855, Adapt transform to C++20 (compilable version) #4888, Adapt transform to C++20 #5051)
• fill (Making fill and fill_n compatible with C++20 #4860)
• find find_end find_first_of find_if find_if_not (Adapting hpx::find and friends to C++20 #4885)
• for_each for_each_n (Add CPOs for for_each #4867)
• for_loop for_loop_strided for_loop_n for_loop_strided_n (Add ranges overloads to for_loop (and variants) #4980)
• generate generate_n (Making generate() and generate_n conforming to C++20 #4896)
• is_heap is_heap_until (Making is_heap and is_heap_until conforming to C++20 #4965)
• is_partitioned (Adapt is_partitioned to C++20 #5006)
• is_sorted is_sorted_until (Adapt is_sorted and is_sorted_until to C++20 #5025)
• lexicographical_compare (Adapt lexicographical_compare to C++20 #5350)
• max_element min_element minmax_element (Adapt min_element, max_element and minmax_element to C++20 #5241)
• make_heap (Adding parallel make_heap  #4964)
• sort (Adapt sort to C++ 20 #5460)
• partial_sort partial_sort_copy (Add partial_sort_copy and adapt partial sort to c++ 20 #5630)
• nth_element (Add nth_element #5592)
• stable_sort (see Adding implementation of stable_sort #4803)
• partition partition_copy stable_partition (Adapt partition, partition_copy and stable_partition to C++ 20 #5540)
• remove remove_if (Adapt remove and remove_if to C++20 #5125)
• remove_copy remove_copy_if (Adapt remove_copy(_if) to C++20 #5150)
• replace replace_copy replace_copy_if replace_if (Adapt replace(_if) to C++20 #5192)
• reverse reverse_copy (Adapt reverse to C++20 #5225)
• rotate rotate_copy (Adapt rotate/rotate_copy to C++20 #5459)
• search (Adapt search to C++20 and Range TS #5066)
• set_difference set_intersection set_symmetric_difference set_union includes (Making set algorithms conform to C++20 #4970)
• merge inplace_merge (Making set algorithms conform to C++20 #4970)
• swap_ranges (Adapt swap_ranges to C++ 20 #5449)
• uninitialized_copy uninitialized_copy_n (Adapt uninitialized_copy and uninitialized_copy_n to C++ 20 #5371)
• uninitialized_fill uninitialized_fill_n (Adapt uninitialized_fill and uninitialized_fill_n to C++ 20 #5402)
• uninitialized_default_construct uninitialized_default_construct_n (Adapt uninitialized_default_construct and uninitialized_default_construct_n to C++ 20 #5415)
• uninitialized_value_construct uninitialized_value_construct_n (Adapt uninitialized_value_construct and uninitialized_value_construct_n to C++ 20 #5416)
• uninitialized_move uninitialized_move_n (Adapt uninitialized_move and uninitialized_move_n to C++ 20 #5389)
• unique unique_copy (Adapt unique and unique_copy to C++ 20 #5458)
• transform_reduce (Making transform_reduce conforming to C++20 #4925)
• transform_exclusive_scan (Adapt transform_exclusive_scan to C++ 20 #5440) transform_inclusive_scan (Adapt transform_inclusive_scan to C++ 20 #5444)
• destroy destroy_n (Making destroy and destroy_n conforming to C++20 #4869)