Add a macro for disabling the C++20 v2 namespace. Add documentation

of the inlining v1/v2 behavior.

doc/intro.qbk

@@ -30,7 +30,7 @@ dramatically.
 provides `std::view_interface`).  These are constrained templates using C++20
 concepts.  These are in the `boost::stl_interfaces::v2` namespace, and are
 considered experimental, because at the time of this writing, no
-C++20-conforming compiler exists.]
+C++20-conforming compiler exists. ]
 
 [heading A Quick Example]
 
@@ -217,3 +217,30 @@ than to try and address the shortcomings of _Iterator_'s `iterator_facade` and
 `iterator_adaptor` templates directly.
 
 [endsect]
+
+[section The `v1` and `v2` Namespaces, and the Use of C++20 Concepts]
+
+For much of this library, there are two interfaces: one in namespace
+`boost::stl_interfaces::v1`, and one in `boost::stl_interfaces::v2`.
+The `v1` version uses SFINAE to constrain templates, and the `v2`
+version uses concepts, including the standard concepts from C++20.
+
+When you build with a conforming C++20 compiler, the compiler defines
+a macro `__cpp_lib_concepts` that indicates that the compiler supports
+concepts, and provides the standard concepts in `std` and
+`std::ranges`.  When _IFaces_ sees the `__cpp_lib_concepts` macro
+defined, it inlines the `v2` namespace; otherwise, it inlines the `v1`
+namespace.
+
+The net result for you as a user is that, for some function or type
+`foo` with `v1` and `v2` versions, you can use
+`boost::stl_interfaces::foo`, and you will get
+`boost::stl_interfaces::v1::foo` when building in C++14 or C++17
+modes, and `boost::stl_interfaces::v2::foo` when building in C++20 or
+later.
+
+If you are using an early C++20 implementation that claims to have
+concept support, but that breaks when you build _IFaces_'s C++20 code,
+you can disable the `v2` namespace by defining _disable_concepts_m_. ]
+
+[endsect]

doc/stl_interfaces.qbk

@@ -61,6 +61,7 @@
 
 [def _concept_m_           [macroref BOOST_STL_INTERFACES_STATIC_ASSERT_CONCEPT]]
 [def _traits_m_            [macroref BOOST_STL_INTERFACES_STATIC_ASSERT_ITERATOR_TRAITS]]
+[def _disable_concepts_m_  [macroref BOOST_STL_INTERFACES_DISABLE_CONCEPTS]]
 
 [def _CRTP_                [@https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern CRTP]]
 [def _Iterator_            [@https://www.boost.org/doc/libs/release/libs/iterator Boost.Iterator]]

include/boost/stl_interfaces/config.hpp

@@ -10,12 +10,18 @@
 #include <iterator>
 
 
+#if defined(__cpp_lib_concepts) && !defined(BOOST_STL_INTERFACES_DISABLE_CONCEPTS)
+#define BOOST_STL_INTERFACES_USE_CONCEPTS 1
+#else
+#define BOOST_STL_INTERFACES_USE_CONCEPTS 0
+#endif
+
 // The inline namespaces v1 and v2 represent pre- and post-C++20.  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 a v2 namespace alternative
 // exists.  Some instances of the v1 namespace may still be inline, if there
 // is no v2 version of its contents.
-#if defined(__cpp_lib_concepts)
+#if BOOST_STL_INTERFACES_USE_CONCEPTS
 #    define BOOST_STL_INTERFACES_NAMESPACE_V1 namespace v1
 #    define BOOST_STL_INTERFACES_NAMESPACE_V2 inline namespace v2
 #else

include/boost/stl_interfaces/fwd.hpp

@@ -8,7 +8,7 @@
 
 #include <boost/stl_interfaces/config.hpp>
 
-#if defined(__cpp_lib_concepts)
+#if BOOST_STL_INTERFACES_USE_CONCEPTS
 #include <ranges>
 #endif
 #if defined(__cpp_lib_three_way_comparison)

include/boost/stl_interfaces/iterator_interface.hpp

@@ -46,7 +46,7 @@ namespace boost { namespace stl_interfaces {
         this template implies a copy or move of the underlying object of type
         `T`. */
     template<typename T>
-#if defined(BOOST_STL_INTERFACES_DOXYGEN) || defined(__cpp_lib_concepts)
+#if defined(BOOST_STL_INTERFACES_DOXYGEN) || BOOST_STL_INTERFACES_USE_CONCEPTS
     // clang-format off
         requires std::is_object_v<T>
 #endif
@@ -546,7 +546,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V1 {
 
 }}}
 
-#if defined(BOOST_STL_INTERFACES_DOXYGEN) || defined(__cpp_lib_concepts)
+#if defined(BOOST_STL_INTERFACES_DOXYGEN) || BOOST_STL_INTERFACES_USE_CONCEPTS
 
 namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V2 {
 
@@ -893,7 +893,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V2 {
             diff_t>::value,                                                    \
         "");
 
-#if defined(__cpp_lib_concepts)
+#if BOOST_STL_INTERFACES_USE_CONCEPTS
 #define BOOST_STL_INTERFACES_STATIC_ASSERT_ITERATOR_TRAITS(                    \
     iter, category, concept, value_type, reference, pointer, difference_type)  \
     static_assert(                                                             \

include/boost/stl_interfaces/reverse_iterator.hpp

@@ -63,7 +63,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V1 {
     struct reverse_iterator
         : iterator_interface<
               reverse_iterator<BidiIter>,
-#if defined(__cpp_lib_concepts)
+#if BOOST_STL_INTERFACES_USE_CONCEPTS
               typename boost::stl_interfaces::v2::v2_dtl::iter_concept_t<
                   BidiIter>,
 #else
@@ -170,7 +170,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V1 {
 }}}
 
 
-#if defined(BOOST_STL_INTERFACES_DOXYGEN) || defined(__cpp_lib_concepts)
+#if defined(BOOST_STL_INTERFACES_DOXYGEN) || BOOST_STL_INTERFACES_USE_CONCEPTS
 
 namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V2 {
 

include/boost/stl_interfaces/sequence_container_interface.hpp

@@ -670,7 +670,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V1 {
 
 }}}
 
-#if defined(BOOST_STL_INTERFACES_DOXYGEN) || defined(__cpp_lib_concepts)
+#if defined(BOOST_STL_INTERFACES_DOXYGEN) || BOOST_STL_INTERFACES_USE_CONCEPTS
 
 namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V2 {
 

include/boost/stl_interfaces/view_interface.hpp

@@ -201,7 +201,7 @@ namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V1 {
 }}}
 
 
-#if defined(BOOST_STL_INTERFACES_DOXYGEN) || defined(__cpp_lib_concepts)
+#if defined(BOOST_STL_INTERFACES_DOXYGEN) || BOOST_STL_INTERFACES_USE_CONCEPTS
 
 namespace boost { namespace stl_interfaces { BOOST_STL_INTERFACES_NAMESPACE_V2 {