Remove BOOST_BEAST_USE_STD_STRING_VIEW

Fixes boostorg#2363

appveyor.yml

@@ -47,8 +47,8 @@ build: off
 test_script:
   - b2 --debug-configuration variant=release cxxstd=14 address-model=64 toolset=msvc-14.0 libs/beast/example -j3
   - b2 --debug-configuration variant=release cxxstd=14 address-model=64 toolset=msvc-14.0 --verbose-test libs/beast/test//run-fat-tests -j3
-  - b2 --debug-configuration variant=release cxxstd=17 define="BOOST_BEAST_USE_STD_STRING_VIEW" address-model=64 toolset=msvc-14.1 libs/beast/example -j3
-  - b2 --debug-configuration variant=release cxxstd=17 define="BOOST_BEAST_USE_STD_STRING_VIEW" address-model=64 toolset=msvc-14.1 --verbose-test libs/beast/test//run-fat-tests -j3
+  - b2 --debug-configuration variant=release cxxstd=17 address-model=64 toolset=msvc-14.1 libs/beast/example -j3
+  - b2 --debug-configuration variant=release cxxstd=17 address-model=64 toolset=msvc-14.1 --verbose-test libs/beast/test//run-fat-tests -j3
 
 cache:
 - c:\tools\vcpkg\installed\

azure-pipelines.yml

@@ -42,7 +42,6 @@ jobs:
           CXX: g++-9
           PACKAGES: g++-9
           VARIANT: release
-          B2_FLAGS: <define>BOOST_BEAST_USE_STD_STRING_VIEW
           CXXSTD: 17
           B2_TARGETS: libs/beast/test//run-fat-tests libs/beast/example
         GCC 9 C++11 HEADER_ONLY NO_DEPRECATED:
@@ -86,7 +85,6 @@ jobs:
           CXX: g++-8
           PACKAGES: g++-8
           VARIANT: release
-          B2_FLAGS: <define>BOOST_BEAST_USE_STD_STRING_VIEW
           CXXSTD: 17
           B2_TARGETS: libs/beast/test//run-fat-tests libs/beast/example
         GCC 8 C++11 HEADER_ONLY NO_DEPRECATED:
@@ -303,13 +301,11 @@ jobs:
         MSVC14.2 C++17 x64:
           VM_IMAGE: 'windows-2019'
           TOOLSET: msvc-14.2
-          B2_FLAGS: define=BOOST_BEAST_USE_STD_STRING_VIEW
           CXXSTD: 17
           ADDRMODEL: 64
         MSVC14.1 C++17 x64:
           VM_IMAGE: 'vs2017-win2016'
           TOOLSET: msvc-14.1
-          B2_FLAGS: define=BOOST_BEAST_USE_STD_STRING_VIEW
           CXXSTD: 17
           ADDRMODEL: 64
         MSVC14.0 C++11 x64:

doc/qbk/03_core/8_conf_macros.qbk

@@ -18,14 +18,6 @@ failure to do so may result in violations of ODR (One Definition Rule).
 
 [table Special Fields
 [[Definition][Description]]
-[
-    [
-        BOOST_BEAST_USE_STD_STRING_VIEW
-    ][
-        Causes Beast to use std::string_view instead of boost::string_view.
-        Requires C++17.
-    ]
-]
 [
     [
         BOOST_BEAST_SEPARATE_COMPILATION

doc/qbk/release_notes.qbk

@@ -15,6 +15,7 @@
 
 [*Miscellaneous]
 
+* [issue 2363] Remove `BOOST_BEAST_USE_STD_STRING_VIEW`
 * [issue 2417] use boost::core::string_view. This improves inter-conversion between string_view implementations. Some observable differences for users:
  * `core::string_view` no longer supports the `.to_string()` or `.clear()` extensions from Utility
  * code that relied on `.max_size()` returning `.size(),` needs to be fixed to use `.size()` instead

include/boost/beast/core/string_type.hpp

@@ -12,32 +12,18 @@
 
 #include <boost/beast/core/detail/config.hpp>
 
-#if defined(BOOST_BEAST_USE_STD_STRING_VIEW)
-#include <string_view>
-#else
 #include <boost/core/detail/string_view.hpp>
-#endif
 
 namespace boost {
 namespace beast {
 
-#if BOOST_BEAST_DOXYGEN || ! defined(BOOST_BEAST_USE_STD_STRING_VIEW)
 /// The type of string view used by the library
 using string_view = boost::core::string_view;
 
 /// The type of `basic_string_view` used by the library
-template<class CharT, class Traits>
-using basic_string_view =
-    boost::basic_string_view<CharT, Traits>;
-
-#else
-using string_view = std::string_view;
-
-template<class CharT, class Traits>
+template<class CharT>
 using basic_string_view =
-    std::basic_string_view<CharT, Traits>;
-
-#endif
+    boost::core::basic_string_view<CharT>;
 
 template<class S>
 inline string_view