Fix standalone compilation error with std::string_view

fixes #1913

CHANGELOG.md

@@ -1,3 +1,4 @@
+* Fix standalone compilation error with std::string_view 
 * OpenSSL 1.0.2 or later is required
 * Fix c++20 deprecation warning in span_body
 

include/boost/beast/http/field.hpp

@@ -396,12 +396,9 @@ field
 string_to_field(string_view s);
 
 /// Write the text for a field name to an output stream.
-inline
+BOOST_BEAST_DECL
 std::ostream&
-operator<<(std::ostream& os, field f)
-{
-    return os << to_string(f);
-}
+operator<<(std::ostream& os, field f);
 
 } // http
 } // beast

include/boost/beast/http/impl/field.ipp

@@ -11,10 +11,12 @@
 #define BOOST_BEAST_HTTP_IMPL_FIELD_IPP
 
 #include <boost/beast/http/field.hpp>
+#include <boost/assert.hpp>
 #include <algorithm>
 #include <array>
 #include <cstring>
-#include <boost/assert.hpp>
+#include <ostream>
+
 
 namespace boost {
 namespace beast {
@@ -565,6 +567,12 @@ string_to_field(string_view s)
     return detail::get_field_table().string_to_field(s);
 }
 
+std::ostream&
+operator<<(std::ostream& os, field f)
+{
+    return os << to_string(f);
+}
+
 } // http
 } // beast
 } // boost

test/beast/http/CMakeLists.txt

@@ -26,6 +26,7 @@ add_executable (tests-beast-http
     empty_body.cpp
     error.cpp
     field.cpp
+    field_compiles.cpp
     fields.cpp
     file_body.cpp
     message.cpp

test/beast/http/Jamfile

@@ -16,6 +16,7 @@ local SOURCES =
     dynamic_body.cpp
     error.cpp
     field.cpp
+    field_compiles.cpp
     fields.cpp
     file_body.cpp
     message.cpp

test/beast/http/field_compiles.cpp

@@ -0,0 +1,11 @@
+//
+// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// 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)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+// Test that header file is self-contained.
+#include <boost/beast/http/field.hpp>