Fix deprecated json_pointer/string operator== warning in value()

value(KeyType&&, default) is constrained on is_comparable_with_object_key,
which passes KeyType as a reference. is_comparable's dispatch on
is_json_pointer_of<A, B> only matches a json_pointer as a plain type or a
plain reference, so a const-qualified reference (as produced when KeyType
is deduced from a json_pointer argument) fell through to
is_comparable_no_json_pointer, which instantiates the deprecated
json_pointer/string comparison operators. This made ordered_json's
transparent comparator (and any transparent comparator on a custom string
type) warn under -Wdeprecated-declarations when calling
value(json_pointer, default), even though no such comparison is ever
performed. at() was already fixed for this in #5289, which does not use
is_comparable_with_object_key.

Strip references and cv-qualifiers with uncvref_t before the
is_json_pointer_of dispatch, so any reference-to-json_pointer is
recognized regardless of qualifiers.

Fixes #5664.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/parser_callback_t.md

@@ -100,6 +100,3 @@ the latter case, it is skipped completely, or replaced by `null` if it is the to
 - Added in version 1.0.0.
 - Fixed in version 3.13.0 to also remove discarded values from a parent object; before, discarding an array or a value
   stored under an object key left a discarded member behind, which made the parse result serialize to invalid JSON.
-- Fixed in version 3.13.0 so that discarding an array or object at its start event also hides its content from the
-  callback, as documented above; before, the callback was still called for the content, and the key of every member of
-  a discarded object was kept in memory until the parse ended.

include/nlohmann/detail/input/json_sax.hpp

@@ -582,8 +582,8 @@ class json_sax_dom_callback_parser
 
     bool start_object(std::size_t len)
     {
-        // check callback for object start; not called inside a discarded container
-        const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
+        // check callback for object start
+        const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
         keep_stack.push_back(keep);
 
         // the key this object will be stored under, read before handle_value()
@@ -619,18 +619,6 @@ class json_sax_dom_callback_parser
 
     bool key(string_t& val)
     {
-        if (!keep_stack.back() || !ref_stack.back())
-        {
-            // the object is not stored: the value of this key is dropped in
-            // handle_value() without touching the key stacks
-            if (keep_stack.back())
-            {
-                BasicJsonType k = BasicJsonType(val);
-                static_cast<void>(callback(static_cast<int>(ref_stack.size()), parse_event_t::key, k));
-            }
-            return true;
-        }
-
         BasicJsonType k = BasicJsonType(val);
 
         // check callback for the key
@@ -716,7 +704,7 @@ class json_sax_dom_callback_parser
 
     bool start_array(std::size_t len)
     {
-        const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
+        const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
         keep_stack.push_back(keep);
 
         // see start_object()

include/nlohmann/detail/meta/type_traits.hpp

@@ -725,7 +725,7 @@ std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::
 // avoid their instantiation on all compilers, even when the first operand
 // is false. The dispatch on is_json_pointer_of can be removed once the
 // deprecated json_pointer comparison operators have been removed.
-template<typename Compare, typename A, typename B, bool = is_json_pointer_of<A, B>::value>
+template<typename Compare, typename A, typename B, bool = is_json_pointer_of<uncvref_t<A>, uncvref_t<B>>::value>
 struct is_comparable : std::false_type {};
 
 template<typename Compare, typename A, typename B>

single_include/nlohmann/json.hpp

@@ -4735,7 +4735,7 @@ std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::
 // avoid their instantiation on all compilers, even when the first operand
 // is false. The dispatch on is_json_pointer_of can be removed once the
 // deprecated json_pointer comparison operators have been removed.
-template<typename Compare, typename A, typename B, bool = is_json_pointer_of<A, B>::value>
+template<typename Compare, typename A, typename B, bool = is_json_pointer_of<uncvref_t<A>, uncvref_t<B>>::value>
 struct is_comparable : std::false_type {};
 
 template<typename Compare, typename A, typename B>
@@ -11996,8 +11996,8 @@ class json_sax_dom_callback_parser
 
     bool start_object(std::size_t len)
     {
-        // check callback for object start; not called inside a discarded container
-        const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
+        // check callback for object start
+        const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
         keep_stack.push_back(keep);
 
         // the key this object will be stored under, read before handle_value()
@@ -12033,18 +12033,6 @@ class json_sax_dom_callback_parser
 
     bool key(string_t& val)
     {
-        if (!keep_stack.back() || !ref_stack.back())
-        {
-            // the object is not stored: the value of this key is dropped in
-            // handle_value() without touching the key stacks
-            if (keep_stack.back())
-            {
-                BasicJsonType k = BasicJsonType(val);
-                static_cast<void>(callback(static_cast<int>(ref_stack.size()), parse_event_t::key, k));
-            }
-            return true;
-        }
-
         BasicJsonType k = BasicJsonType(val);
 
         // check callback for the key
@@ -12130,7 +12118,7 @@ class json_sax_dom_callback_parser
 
     bool start_array(std::size_t len)
     {
-        const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
+        const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
         keep_stack.push_back(keep);
 
         // see start_object()

tests/src/unit-class_parser.cpp

@@ -1966,102 +1966,6 @@ TEST_CASE("parser class")
             }
         }
 
-        SECTION("no callback for the content of a discarded container (#5643)")
-        {
-            // discarding a container at its start event must also hide
-            // everything inside it from the callback: none of the nested
-            // keys, values, or nested containers' own start/end events may
-            // be reported
-            std::vector<std::string> log;
-            bool first = true;
-            const json j = json::parse(R"({"skip": {"k1": 1, "k2": [2, {"k3": 3}]}, "keep": 1})",
-                                       [&](int depth, json::parse_event_t event, json & parsed)
-            {
-                static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"};
-                log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());
-
-                if (depth == 1 && event == json::parse_event_t::object_start && first)
-                {
-                    // discard "skip" right at its object_start event
-                    first = false;
-                    return false;
-                }
-                return true;
-            });
-
-            CHECK(log == std::vector<std::string>
-            {
-                "0 object_start <discarded>",
-                "1 key \"skip\"",
-                "1 object_start <discarded>",
-                "1 key \"keep\"",
-                "1 value 1",
-                "0 object_end {\"keep\":1}"
-            });
-            CHECK(j == json({{"keep", 1}}));
-        }
-
-        SECTION("callback still called inside a container whose key was rejected (#5643)")
-        {
-            // rejecting a key does not discard its value's container at the
-            // container's own start event, so the callback is still called
-            // for that container's content; only storing the container
-            // under the rejected key is skipped
-            // (documented for parser_callback_t: "the callback is still
-            // called for the associated value, but its return value has no
-            // further effect")
-            const auto record = [](std::vector<std::string>& log, int depth, json::parse_event_t event, const json & parsed)
-            {
-                static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"};
-                log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());
-            };
-
-            std::vector<std::string> log_object;
-            const json j_object = json::parse(R"({"skip": {"k1": 1}, "keep": 2})",
-                                              [&](int depth, json::parse_event_t event, json & parsed)
-            {
-                record(log_object, depth, event, parsed);
-                return !(event == json::parse_event_t::key && parsed == json("skip"));
-            });
-
-            CHECK(log_object == std::vector<std::string>
-            {
-                "0 object_start <discarded>",
-                "1 key \"skip\"",
-                "1 object_start <discarded>",
-                "2 key \"k1\"",
-                "2 value 1",
-                "1 key \"keep\"",
-                "1 value 2",
-                "0 object_end {\"keep\":2}"
-            });
-            CHECK(j_object == json({{"keep", 2}}));
-
-            // same for a rejected key whose value is an array rather than an object
-            std::vector<std::string> log_array;
-            const json j_array = json::parse(R"({"skip": [1, {"k1": 2}], "keep": 2})",
-                                             [&](int depth, json::parse_event_t event, json & parsed)
-            {
-                record(log_array, depth, event, parsed);
-                return !(event == json::parse_event_t::key && parsed == json("skip"));
-            });
-
-            CHECK(log_array == std::vector<std::string>
-            {
-                "0 object_start <discarded>",
-                "1 key \"skip\"",
-                "1 array_start <discarded>",
-                "2 value 1",
-                "2 object_start <discarded>",
-                "3 key \"k1\"",
-                "3 value 2",
-                "1 key \"keep\"",
-                "1 value 2",
-                "0 object_end {\"keep\":2}"
-            });
-            CHECK(j_array == json({{"keep", 2}}));
-        }
-
         SECTION("special cases")
         {
             // the following test cases cover the situation in which an empty

tests/src/unit-json_pointer.cpp

@@ -858,6 +858,25 @@ TEST_CASE("JSON pointers")
         }
     }
 
+    SECTION("value(json_pointer, default) with ordered_json #5664")
+    {
+        // ordered_json's transparent object comparator made value()'s
+        // is_comparable_with_object_key check (which passes the pointer as
+        // a reference) instantiate the deprecated json_pointer/string
+        // comparison; this must compile without relying on it. The
+        // deprecation warning itself is not observable here, since the
+        // unit test build disables -Wdeprecated-declarations (see
+        // cmake/clang_flags.cmake); it was checked manually instead.
+        const nlohmann::ordered_json j = {{"n", 1}, {"s", "text"}};
+        const nlohmann::ordered_json::json_pointer ptr_n("/n");
+        const nlohmann::ordered_json::json_pointer ptr_s("/s");
+        const nlohmann::ordered_json::json_pointer ptr_missing("/missing");
+
+        CHECK(j.value(ptr_n, 0) == 1);
+        CHECK(j.value(ptr_s, std::string("x")) == "text");
+        CHECK(j.value(ptr_missing, 42) == 42);
+    }
+
     // build with C++20
     // JSON_HAS_CPP_20
 #if defined(__cpp_char8_t)