dense_map: transparent lookup for at (#1270)

src/entt/container/dense_map.hpp

@@ -725,6 +725,24 @@ public:
         return it->second;
     }
 
+    /*! @copydoc at */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, mapped_type const &>>
+    at(const Other &key) const {
+        auto it = find(key);
+        ENTT_ASSERT(it != cend(), "Invalid key");
+        return it->second;
+    }
+
+    /*! @copydoc at */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, mapped_type &>>
+    at(const Other &key) {
+        auto it = find(key);
+        ENTT_ASSERT(it != end(), "Invalid key");
+        return it->second;
+    }
+
     /**
      * @brief Accesses or inserts a given element.
      * @param key A key of an element to find or insert.

test/entt/container/dense_map.cpp

@@ -66,6 +66,9 @@ TEST(DenseMap, Functionalities) {
 
     map.emplace(0, 0);
 
+    ASSERT_EQ(map.at(0), 0); // regular key lookup
+    ASSERT_EQ(map.at(0.0), 0); // transparent key lookup
+
     ASSERT_EQ(map.count(0), 1u);
     ASSERT_EQ(map.count(6.4), 0u);
     ASSERT_EQ(cmap.count(0.0), 1u);