Improved SFINAE in the JSON encoders

doc/exception.adoc

@@ -1084,8 +1084,9 @@ struct boost_json_encoder
 {
     boost::json::value & v_;
 
-    // Enabled if x is assignable to boost::json::value, or
+    // Uses unspecified SFINAE expression designed to make the
-    // if tag_invoke is defined for boost::json::value_from_tag.
+    // overload selected only if no other compatible overload is found.
+    // Implemented in terms of boost::json::value_from.
     template <class T>
     friend void output( boost_json_encoder &, T const & x );
 
@@ -1114,7 +1115,9 @@ struct nlohmann_json_encoder
 {
     Json & j_;
 
-    // Enabled if to_json is available for Json and T.
+    // Uses unspecified SFINAE expression designed to make the
+    // overload selected only if no other compatible overload is found.
+    // Implemented in terms of to_json.
     template <class T>
     friend void output( nlohmann_json_encoder &, T const & x );
 
@@ -1156,8 +1159,9 @@ struct boost_json_encoder
 {
     boost::json::value & v_;
 
-    // Enabled if x is assignable to boost::json::value, or
+    // Uses unspecified SFINAE expression designed to make the
-    // if tag_invoke is defined for boost::json::value_from_tag.
+    // overload selected only if no other compatible overload is found.
+    // Implemented in terms of boost::json::value_from.
     template <class T>
     friend void output( boost_json_encoder &, T const & x );
 
@@ -1168,10 +1172,7 @@ struct boost_json_encoder
 } }
 ----
 
-The `boost_json_encoder` type serializes objects to JSON format using https://www.boost.org/doc/libs/release/libs/json/[Boost.JSON]. The `output` function is enabled for:
+The `boost_json_encoder` type serializes objects to JSON format using https://www.boost.org/doc/libs/release/libs/json/[Boost.JSON]. The `output` function is implemented in terms of `boost::json::value_from`.
-
-* Types directly assignable to `boost::json::value`
-* Types for which a `tag_invoke` overload for `value_from_tag` can be found via ADL
 
 See <<tutorial_serialization>>.
 
@@ -1592,7 +1593,9 @@ struct nlohmann_json_encoder
 {
     Json & j_;
 
-    // Enabled if to_json is available for Json and T.
+    // Uses unspecified SFINAE expression designed to make the
+    // overload selected only if no other compatible overload is found.
+    // Implemented in terms of to_json.
     template <class T>
     friend void output( nlohmann_json_encoder &, T const & x );
 
@@ -1603,7 +1606,9 @@ struct nlohmann_json_encoder
 } }
 ----
 
-The `nlohmann_json_encoder` type serializes objects to JSON format based on ADL calls to `to_json`. This is compatible with https://github.com/nlohmann/json[nlohmann/json]. See <<tutorial_serialization>>.
+The `nlohmann_json_encoder` type serializes objects to JSON format based on ADL calls to `to_json`. This is compatible with https://github.com/nlohmann/json[nlohmann/json].
+
+See <<tutorial_serialization>>.
 
 '''
 

include/boost/exception/serialization/boost_json_encoder.hpp

@@ -6,43 +6,36 @@
 #ifndef BOOST_EXCEPTION_SERIALIZATION_BOOST_JSON_ENCODER_HPP_INCLUDED
 #define BOOST_EXCEPTION_SERIALIZATION_BOOST_JSON_ENCODER_HPP_INCLUDED
 
+#include <type_traits>
 #include <utility>
 
+namespace boost { namespace json {
+
+class value;
+
+template <class T>
+void value_from(T &&, value &);
+
+} }
+
 namespace
 boost
     {
-    namespace
-    json
-        {
-        class value;
-        struct value_from_tag;
-        }
-
     namespace
     exception_serialization
         {
-        // Baast.JSON does not provide ADL interface for serializing user-defined types.
+        template <class Value = boost::json::value>
-        // This limits the functionality of boost_json_encoder to only types that provide tag_invoke.
-        template <class Value = boost::json::value, class ValueFromTag = boost::json::value_from_tag>
         struct
         boost_json_encoder_
             {
             Value & v_;
 
-            template <class T>
+            template <class Encoder, class T, class... Deprioritize>
             friend
-            auto
+            typename std::enable_if<std::is_same<Encoder, boost_json_encoder_>::value>::type
-            output(boost_json_encoder_ & e, T const & x) -> decltype(std::declval<Value &>() = x, void())
+            output(Encoder & e, T const & x, Deprioritize...)
                 {
-                e.v_ = x;
+                boost::json::value_from(x, e.v_);
-                }
-
-            template <class T>
-            friend
-            auto
-            output(boost_json_encoder_ & e, T const & x) -> decltype(tag_invoke(std::declval<ValueFromTag>(), std::declval<Value &>(), x), void())
-                {
-                tag_invoke(ValueFromTag{}, e.v_, x);
                 }
 
             template <class T>

include/boost/exception/serialization/nlohmann_json_encoder.hpp

@@ -6,6 +6,7 @@
 #ifndef BOOST_EXCEPTION_SERIALIZATION_NLOHMANN_JSON_ENCODER_HPP_INCLUDED
 #define BOOST_EXCEPTION_SERIALIZATION_NLOHMANN_JSON_ENCODER_HPP_INCLUDED
 
+#include <type_traits>
 #include <utility>
 
 namespace
@@ -20,12 +21,12 @@ boost
             {
             Json & j_;
 
-            template <class T>
+            template <class Encoder, class T, class... Deprioritize>
-            friend
+            friend typename std::enable_if<std::is_same<Encoder, nlohmann_json_encoder>::value, Json *>::type
-            auto
+            output(Encoder & e, T const & x, Deprioritize...)
-            output(nlohmann_json_encoder & e, T const & x) -> decltype(to_json(std::declval<Json &>(), x))
                 {
                 to_json(e.j_, x);
+                return 0;
                 }
 
             template <class T>

test/boost_json_test.cpp

@@ -18,6 +18,7 @@
 #include <iomanip>
 #include <iostream>
 #include <exception>
+#include <vector>
 
 using output_encoder = boost::exception_serialization::boost_json_encoder;
 
@@ -39,6 +40,7 @@ boost
 
 typedef boost::error_info<struct my_error1_, int> my_error1;
 typedef boost::error_info<struct my_error2_, std::string> my_error2;
+typedef boost::error_info<struct my_error4_, std::vector<int>> my_error4;
 
 struct
 my_info
@@ -97,6 +99,13 @@ check_output(boost::json::value const & j, bool has_source_location)
     auto const & mij = obj.at("my_error3_").as_object();
     BOOST_TEST_EQ(mij.at("code").as_int64(), 1);
     BOOST_TEST_EQ(mij.at("message").as_string(), "error one");
+
+    BOOST_TEST(obj.contains("my_error4_"));
+    auto const & vec = obj.at("my_error4_").as_array();
+    BOOST_TEST_EQ(vec.size(), 3u);
+    BOOST_TEST_EQ(vec[0].as_int64(), 1);
+    BOOST_TEST_EQ(vec[1].as_int64(), 2);
+    BOOST_TEST_EQ(vec[2].as_int64(), 3);
     }
 
 int
@@ -111,7 +120,8 @@ main()
             e <<
                 my_error1(42) <<
                 my_error2("hello") <<
-                my_error3({1, "error one"});
+                my_error3({1, "error one"}) <<
+                my_error4({1, 2, 3});
             BOOST_THROW_EXCEPTION(e);
             }
         catch( test_exception & e )
@@ -132,7 +142,8 @@ main()
             e <<
                 my_error1(42) <<
                 my_error2("hello") <<
-                my_error3({1, "error one"});
+                my_error3({1, "error one"}) <<
+                my_error4({1, 2, 3});
             BOOST_THROW_EXCEPTION(e);
             }
         catch( ... )