C++20 "standard" conform

[marehr]

Description

As a Library we want to be C++20 "standard" conform

From the beginning of seqan3 development, we decided to use the ranges library and the language feature of concepts that we knew that will be part of the C++ standard at one point.

Initially, we expected it to be in the then upcoming C++17 standard but it was postponed to C++20.

During the standardisation process, some names, syntax and paradigms changed within the ranges and concept proposal. Thus, this epic issue will keep track of those changes to make the seqan3 library "standard" conform.

Acceptance Criteria

• The seqan3 library is C++20 standard conform and
• compiles on linux with C++20 compatible compilers, e.g. gcc and clang.

Tasks

• Requires clauses need parenthesis for non-primary expressions Requires clauses need parenthesis for non-primary expressions #53
• return-type-requirement is not a type-constraint cxx20: return-type-requirement is not a type-constraint  #88
• std::span should have signature std::span<typename span_tp, size_t span_extent> instead of std::span<typename span_tp, ptrdiff_t span_extent> [MISC] Use correct definition of std::span<T, E> (extent should be std::size_t) seqan3#1712
• Use internally std::(ranges::range|iter)_(size|difference|value|reference|rvalue_reference)_t instead of seqan3::(size_type|difference_type|value_type|reference|rvalue_reference|const_reference)_t. Deprecate type_traits/iterator.hpp seqan3#1549
• Update Ranges-Library to keep track of changes in the standard [MISC] Update range library seqan3#1702
• Remove dependencies of seqan3 in seqan3/std/* headers Remove dependencies of seqan3 in seqan3/std/* headers #135
• Remove "Associated types" from views Remove "Associated Types" from views #51
• Use std:: versions of views and ranges instead of ranges-v3
  • std::ranges::view_base instead of ::ranges::view_base [FIX] use std::ranges::view_base instead of ranges one seqan3#1906
  • std::ranges::ref_view instead of ::ranges::ref_view [FIX] use std::ranges::ref_view instead of ranges one seqan3#1905
• Move C++20 things into std namespace Move C++20 things into std namespace #134
• Introduce seqan3/std/bit and deprecate most of seqan3/core/bit_manipulation.hpp Introduce seqan3/std/bit and deprecate most of seqan3/core/bit_manipulation.hpp #199
• Stuff that was renamed by the standard or was in the wrong namespace or was exposition-only:
  • Rename ranges::enable_safe_range as ranges::enable_borrowed_range. Rename enable_safe_range to enable_borrowed_range. #48
  • Replace seqan3::forwarding_range by std::ranges::borrowed_range. [FIX] use std::ranges::borrowed_range instead of seqan3::forwarding_range seqan3#2038
  • std::default_constructible -> std::default_initializable
  • std::boolean -> this is exposition-only and called boolean-testable; either throw all usages away or implement it in seqan3 namespace Simplify seqan3/std/concepts seqan3#2023
  • std::detail::weakly_equality_comparable_with -> this is exposition-only and called weakly-equality-comparable-with; either throw all usages away or implement it in seqan3 namespace (fun-fact it is already implemented in include/seqan3/core/concept/core_language.hpp) Simplify seqan3/std/concepts seqan3#2023
  • std::readable -> std::indirectly_readable
  • std::writable -> std::indirectly_writable
  • std::back_inserter -> std::cpp20::back_inserter (back_inserter has a new behaviour since cxx20; it is default initializable)
  • std::ranges::default_sentinel -> std::default_sentinel (we wrongly assume that these are in std::ranges namespace, even though they are in std namespace)
  • std::ranges::default_sentinel_t -> std::default_sentinel_t (we wrongly assume that these are in std::ranges namespace, even though they are in std namespace)
  • std::ranges::all_view -> std::views::all_t
  • Remove iterator related stuff from std::ranges when in C++17 mode. (std::forward_iterator, ...)
    • Explicitly import every single entity into std namespace [FIX] explicitly import cxx20 things into std namespace seqan3#1828

Definition of Done

• Implementation and design approved
• Unit tests pass
• Test coverage = 100%
• Microbenchmarks added and/or affected microbenchmarks < 5% performance drop
• API documentation added
• Tutorial/teaching material added
• Test suite compiles in less than 30 seconds (on travis)
• Changelog entry added

[marehr]

This is done; since gcc-10 and gcc-11 is C++20 conforming enough for us.

Other compiler do not work yet, because of some major pending bugs. See #127 for example.