allow forward declaration of has_ traits

doc/pages/conversion/forward.adoc

@@ -41,3 +41,18 @@ Forward declarations of contextual conversions are done very similarly:
 ----
 include::../../../test/doc_forward_conversion.cpp[tag=doc_forward_conversion_3,indent=0]
 ----
+
+
+== Checking for Conversion Support
+
+In some cases it might be necessary for you to check if a type is supported by
+JSON's conversion functions. For example, you want to provide a fallback
+overload for types that do not have such support. The type traits
+<<ref_has_value_from>> and <<ref_has_value_to>> are intended for such checks.
+They can also be forward-declared in case a physical dependency on JSON is
+not desirable.
+
+[source]
+----
+include::../../../test/doc_forward_conversion.cpp[tag=doc_forward_conversion_4,indent=0]
+----

include/boost/json/fwd.hpp

@@ -84,6 +84,12 @@ template<class T>
 typename result_for<T, value>::type
 result_from_errno( int e, boost::source_location const* loc ) noexcept;
 
+template<class T>
+struct has_value_from;
+
+template<class T>
+struct has_value_to;
+
 #endif
 
 } // namespace json

include/boost/json/value_from.hpp

@@ -153,17 +153,15 @@ value_from(
     If `T` can be converted to @ref value via a call to @ref value_from, the
     static data member `value` is defined as `true`. Otherwise, `value` is
     defined as `false`.
-
-    @see @ref value_from.
 */
-#ifdef BOOST_JSON_DOCS
-template<class T>
-using has_value_from = __see_below__;
-#else
 template<class T>
-using has_value_from = detail::can_convert<
-    detail::remove_cvref<T>, detail::value_from_conversion>;
+struct has_value_from
+#ifndef BOOST_JSON_DOCS
+    : detail::can_convert<
+        detail::remove_cvref<T>, detail::value_from_conversion>
+{}
 #endif
+;
 
 } // namespace json
 } // namespace boost

include/boost/json/value_to.hpp

@@ -215,21 +215,18 @@ try_value_to(const value& jv)
 
 /** Determine if a @ref value can be converted to `T`.
 
-    If @ref value can be converted to `T` via a
-    call to @ref value_to, the static data member `value`
-    is defined as `true`. Otherwise, `value` is
+    If @ref value can be converted to `T` via a call to @ref value_to, the
+    static data member `value` is defined as `true`. Otherwise, `value` is
     defined as `false`.
-
-    @see @ref value_to
 */
-#ifdef BOOST_JSON_DOCS
-template<class T>
-using has_value_to = __see_below__;
-#else
 template<class T>
-using has_value_to = detail::can_convert<
-    detail::remove_cvref<T>, detail::value_to_conversion>;
+struct has_value_to
+#ifndef BOOST_JSON_DOCS
+    : detail::can_convert<
+        detail::remove_cvref<T>, detail::value_to_conversion>
+{}
 #endif
+;
 
 } // namespace json
 } // namespace boost

test/doc_forward_conversion.cpp

@@ -32,6 +32,20 @@ try_value_to( const value& jv );
 }
 // end::doc_forward_conversion_1[]
 
+// tag::doc_forward_conversion_4[]
+namespace boost {
+namespace json {
+
+template< class T >
+struct has_value_from;
+
+template< class T >
+struct has_value_to;
+
+}
+}
+// end::doc_forward_conversion_4[]
+
 // tag::doc_forward_conversion_3[]
 namespace boost {
 namespace json {
@@ -156,6 +170,35 @@ tag_invoke(
 }
 // end::doc_forward_conversion_3[]
 
+// tag::doc_forward_conversion_4[]
+
+namespace third_party
+{
+
+struct json_storage
+{
+    boost::json::value& jv;
+
+    template< class T >
+    friend
+    typename std::enable_if< boost::json::has_value_from<T>::value >::type
+    process(json_storage& s, T const& t)
+    {
+        boost::json::value_from(t, s.jv);
+    }
+
+    template< class T >
+    friend
+    typename std::enable_if< ! boost::json::has_value_from<T>::value >::type
+    process(json_storage& s, T const&)
+    {
+        boost::json::value_from("unknown value", s.jv);
+    }
+};
+
+}
+// end::doc_forward_conversion_4[]
+
 #include <boost/json/value_from.hpp>
 #include <boost/json/value_to.hpp>
 
@@ -193,6 +236,16 @@ class doc_forward_conversion
         BOOST_TEST( get<1>(addr2) == 115 );
         BOOST_TEST( get<2>(addr2) == 81 );
         BOOST_TEST( get<3>(addr2) == 22 );
+
+        jv.emplace_null();
+        third_party::json_storage s{jv};
+        process(s, addr2);
+        BOOST_TEST(( jv == value{ 212, 115, 81, 22 } ));
+
+        struct some_struct {};
+        jv.emplace_null();
+        process(s, some_struct{});
+        BOOST_TEST(( jv == value{ "unknown value" } ));
     }
 };