Add stl_interfaces fallback when deducing this is unavailable.

Added a build option, BEMAN_ITERATOR_INTERFACE26_USE_DEDUCING_THIS, that
switches between the deducing this implementation and that used by
stl_interfaces. It defaults to whether the compiler supports deducing this.

This element of configuration is placed in a generated config.hpp. This
was chosen, as opposed to using a define passed as a compiler flag, to
avoid ODR violations with larger projects with incoherent flag usage.

The `iterator_interface_access` struct was moved to its own header since
it is needed by both the deducing this and stl_interfaces implementations.
The stl_interfaces implementation was modifed to use this so it better
conforms to the paper.

Some cleanup is still needed before this gets merged in:

- There are optional26 remnants where stl_interfaces was brought
  in from.
- Documentation is needed to explain when the extra template parameter
  is required. BEMAN_ITERATOR_INTERFACE26_USE_DEDUCING_THIS() should
  probably also be mentioned in that context.

The intent is to address these once the overall approach has
consensus.

Fixes beman-project#10 and depends on beman-project#12.

CMakeLists.txt

@@ -8,10 +8,20 @@ include(CompilerFeatureTest)
 
 beman_iterator26_check_deducing_this(COMPILER_SUPPORTS_DEDUCING_THIS)
 
-if(NOT COMPILER_SUPPORTS_DEDUCING_THIS)
-  message(FATAL_ERROR "The selected compiler and flags lack C++23's deducing this support, which is required to build this project. Try adding -DCMAKE_CXX_STANDARD=23 to your command line parameters and, failing that, upgrade your compiler.")
+option(BEMAN_ITERATOR_INTERFACE26_USE_DEDUCING_THIS
+  "Make use of deducing this. Turn this off for non-conforming compilers."
+  ${COMPILER_SUPPORTS_DEDUCING_THIS})
+
+if(BEMAN_ITERATOR_INTERFACE26_USE_DEDUCING_THIS AND NOT COMPILER_SUPPORTS_DEDUCING_THIS)
+  message(WARNING "Building with deducing this support despite of the compiler's lack of support for it")
 endif()
 
+configure_file(
+  "${PROJECT_SOURCE_DIR}/include/beman/iterator_interface26/config.hpp.in"
+  "${PROJECT_BINARY_DIR}/include/beman/iterator_interface26/config.hpp"
+  @ONLY
+)
+
 enable_testing()
 
 set(TARGETS_EXPORT_NAME ${CMAKE_PROJECT_NAME}Targets)

include/beman/iterator_interface26/config.hpp.in

@@ -0,0 +1,6 @@
+#ifndef BEMAN_ITERATOR_INTERFACE26_CONFIG_HPP
+#define BEMAN_ITERATOR_INTERFACE26_CONFIG_HPP
+
+#cmakedefine01 BEMAN_ITERATOR_INTERFACE26_USE_DEDUCING_THIS()
+
+#endif

include/beman/iterator_interface26/detail/stl_interfaces/config.hpp

@@ -0,0 +1,49 @@
+// include/beman/optional26/detail/stl_interfaces/config.hpp -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Copyright (C) 2020 T. Zachary Laine
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+#ifndef BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_CONFIG_HPP
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_CONFIG_HPP
+
+// Included for definition of __cpp_lib_concepts.
+#include <iterator>
+
+#if defined(__cpp_lib_concepts) && defined(__cpp_lib_ranges) && \
+    !defined(BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_DISABLE_CONCEPTS)
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS 1
+#else
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS 0
+#endif
+
+#if defined(__cpp_explicit_this_parameter) && BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS && \
+    !defined(BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_DISABLE_DEDUCED_THIS)
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_DEDUCED_THIS 1
+#else
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_DEDUCED_THIS 0
+#endif
+
+// The inline namespaces v1, v2, and v3 represent C++14, C++20, and C++23 and
+// later, respectively.  v1 is inline for standards before C++20, and v2 is
+// inline for C++20 and later.  Note that this only applies to code for which
+// multiple vI namespace alternatives exist.  For example, some instances of
+// the v1 namespace may still be inline, if there is no v2 version of its
+// contents.
+#if !BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS && !BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_DEDUCED_THIS
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V1 inline namespace v1
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V2 namespace v2
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V3 namespace v3
+#elif BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS && !BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_DEDUCED_THIS
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V1 namespace v1
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V2 inline namespace v2
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V3 namespace v3
+#else
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V1 namespace v1
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V2 namespace v2
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V3 inline namespace v3
+#endif
+
+#endif

include/beman/iterator_interface26/detail/stl_interfaces/fwd.hpp

@@ -0,0 +1,88 @@
+// include/beman/optional26/detail/stl_interfaces/fwd.hpp -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Copyright (C) 2019 T. Zachary Laine
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+#ifndef BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_FWD_HPP
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_FWD_HPP
+
+#include <beman/iterator_interface26/detail/stl_interfaces/config.hpp>
+
+#if BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_USE_CONCEPTS
+#include <ranges>
+#endif
+#if defined(__cpp_lib_three_way_comparison)
+#include <compare>
+#endif
+
+#ifndef BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_DOXYGEN
+
+#if defined(_MSC_VER) || defined(__GNUC__) && __GNUC__ < 8
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NO_HIDDEN_FRIEND_CONSTEXPR
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_HIDDEN_FRIEND_CONSTEXPR
+#else
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_HIDDEN_FRIEND_CONSTEXPR constexpr
+#endif
+
+#if defined(__GNUC__) && __GNUC__ < 9
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_CONCEPT concept bool
+#else
+#define BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_CONCEPT concept
+#endif
+
+#endif
+
+namespace beman::iterator_interface26::detail {
+namespace stl_interfaces {
+
+/** An enumeration used to indicate whether the underlying data have a
+    contiguous or discontiguous layout when instantiating `view_interface`
+    and `sequence_container_interface`. */
+enum class element_layout : bool { discontiguous = false, contiguous = true };
+
+BEMAN_OPTIONAL26_DETAIL_STL_INTERFACES_NAMESPACE_V1 {
+
+    namespace v1_dtl {
+    template <typename... T>
+    using void_t = void;
+
+    template <typename Iter>
+    using iter_difference_t = typename std::iterator_traits<Iter>::difference_type;
+
+    template <typename Range, typename = void>
+    struct iterator;
+    template <typename Range>
+    struct iterator<Range, void_t<decltype(std::declval<Range&>().begin())>> {
+        using type = decltype(std::declval<Range&>().begin());
+    };
+    template <typename Range>
+    using iterator_t = typename iterator<Range>::type;
+
+    template <typename Range, typename = void>
+    struct sentinel;
+    template <typename Range>
+    struct sentinel<Range, void_t<decltype(std::declval<Range&>().end())>> {
+        using type = decltype(std::declval<Range&>().end());
+    };
+    template <typename Range>
+    using sentinel_t = typename sentinel<Range>::type;
+
+    template <typename Range>
+    using range_difference_t = iter_difference_t<iterator_t<Range>>;
+
+    template <typename Range>
+    using common_range = std::is_same<iterator_t<Range>, sentinel_t<Range>>;
+
+    template <typename Range, typename = void>
+    struct decrementable_sentinel : std::false_type {};
+    template <typename Range>
+    struct decrementable_sentinel<Range, void_t<decltype(--std::declval<sentinel_t<Range>&>())>> : std::true_type {};
+    } // namespace v1_dtl
+}
+} // namespace stl_interfaces
+} // namespace beman::optional26::detail
+
+#endif