Fix element path for map/unordered_map JSON_DIAGNOSTICS errors

When converting a JSON array to std::map or std::unordered_map with a
non-string key, each element must itself be a [key, value] array. If an
element is not an array, from_json() threw type_error 302 with the outer
array's value (&j) as the exception context, so with JSON_DIAGNOSTICS
enabled the message pointed at the whole array instead of the offending
element (e.g. "(/outer/m)" instead of "(/outer/m/2)"), even though the
message text already described the element's type.

Both from_json() overloads now pass the element (&p) as the context, so
the reported JSON Pointer matches the type named in the message, the
same way std::vector<std::vector<T>> and similar conversions already do.

Fixes #5668.

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/conversions/from_json.hpp

@@ -564,7 +564,7 @@ inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allo
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -584,7 +584,7 @@ inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Has
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }

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()

single_include/nlohmann/json.hpp

@@ -6061,7 +6061,7 @@ inline void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allo
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -6081,7 +6081,7 @@ inline void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Has
     {
         if (JSON_HEDLEY_UNLIKELY(!p.is_array()))
         {
-            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &j));
+            JSON_THROW(type_error::create(302, concat("type must be array, but is ", p.type_name()), &p));
         }
         m.emplace(p.at(0).template get<Key>(), p.at(1).template get<Value>());
     }
@@ -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-diagnostics.cpp

@@ -17,6 +17,9 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#include <map>
+#include <unordered_map>
+
 TEST_CASE("Better diagnostics")
 {
     SECTION("empty JSON Pointer")
@@ -331,6 +334,28 @@ TEST_CASE("Regression tests for extended diagnostics")
         }
     }
 
+    SECTION("Regression test for issue #5668 - wrong path for std::map/unordered_map with non-string keys")
+    {
+        // a map with non-string keys is read from an array of [key, value] arrays;
+        // element 2 of "m" is not an array, so the path must point at "m/2", not "m"
+        json j;
+        j["outer"]["m"] = json::array({json::array({1, 2}), json::array({3, 4}), 5});
+
+        SECTION("std::map")
+        {
+            CHECK_THROWS_WITH_AS((j["outer"]["m"].get<std::map<int, int>>()),
+                                 "[json.exception.type_error.302] (/outer/m/2) type must be array, "
+                                 "but is number", json::type_error);
+        }
+
+        SECTION("std::unordered_map")
+        {
+            CHECK_THROWS_WITH_AS((j["outer"]["m"].get<std::unordered_map<int, int>>()),
+                                 "[json.exception.type_error.302] (/outer/m/2) type must be array, "
+                                 "but is number", json::type_error);
+        }
+    }
+
     SECTION("Regression test - swap(array_t&)/swap(object_t&) must update JSON_DIAGNOSTICS parent pointers")
     {
         // swap(array_t&)