Fix and add test's for SFINAE problem

include/nlohmann/json.hpp

@@ -4010,7 +4010,7 @@ class basic_json
     @since version 3.6.0
     */
     template<typename KeyT, typename std::enable_if<
-                 not std::is_same<KeyT, json_pointer>::value, int>::type = 0>
+                 not std::is_same<typename std::decay<KeyT>::type, json_pointer>::value, int>::type = 0>
     bool contains(KeyT && key) const
     {
         return is_object() and m_value.object->find(std::forward<KeyT>(key)) != m_value.object->end();

single_include/nlohmann/json.hpp

@@ -18445,7 +18445,7 @@ class basic_json
     @since version 3.6.0
     */
     template<typename KeyT, typename std::enable_if<
-                 not std::is_same<KeyT, json_pointer>::value, int>::type = 0>
+                 not std::is_same<typename std::decay<KeyT>::type, json_pointer>::value, int>::type = 0>
     bool contains(KeyT && key) const
     {
         return is_object() and m_value.object->find(std::forward<KeyT>(key)) != m_value.object->end();

test/src/unit-regression.cpp

@@ -1809,6 +1809,17 @@ TEST_CASE("regression tests")
         json j = smallest;
         CHECK(j.dump() == std::to_string(smallest));
     }
+
+    SECTION("issue #1727 - Contains with non-const lvalue json_pointer picks the wrong overload")
+    {
+        json j = {{"root", {{"settings", {{"logging", true}}}}}};
+
+        auto jptr1 = "/root/settings/logging"_json_pointer;
+        auto jptr2 = json::json_pointer{"/root/settings/logging"};
+
+        CHECK(j.contains(jptr1));
+        CHECK(j.contains(jptr2));
+    }
 }
 
 #if not defined(JSON_NOEXCEPTION)