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>

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>());
     }

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>());
     }

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&)