Fix deprecated json_pointer/string operator== warning in value() (#5683)

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>

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>

tests/src/unit-json_pointer.cpp

@@ -878,6 +878,25 @@ 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)