registry: allow non-const types in const overloads of registry context variable getters (close #720)

src/entt/entity/registry.hpp

@@ -1633,9 +1633,9 @@ public:
      * registry, a null pointer otherwise.
      */
     template<typename Type>
-    [[nodiscard]] Type * try_ctx() const {
+    [[nodiscard]] std::add_const_t<Type> * try_ctx() const {
         auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
-        return it == vars.cend() ? nullptr : any_cast<Type>(&*it);
+        return it == vars.cend() ? nullptr : any_cast<std::add_const_t<Type>>(&*it);
     }
 
     /*! @copydoc try_ctx */
@@ -1656,10 +1656,10 @@ public:
      * @return A valid reference to the object in the context of the registry.
      */
     template<typename Type>
-    [[nodiscard]] Type & ctx() const {
+    [[nodiscard]] std::add_const_t<Type> & ctx() const {
         auto it = std::find_if(vars.cbegin(), vars.cend(), [type = type_id<Type>()](auto &&var) { return var.type() == type; });
         ENTT_ASSERT(it != vars.cend(), "Invalid instance");
-        return any_cast<Type &>(*it);
+        return any_cast<std::add_const_t<Type> &>(*it);
     }
 
     /*! @copydoc ctx */

test/entt/entity/registry.cpp

@@ -1751,20 +1751,32 @@ TEST(Registry, GetOrEmplace) {
 TEST(Registry, Constness) {
     entt::registry registry;
 
-    ASSERT_TRUE((std::is_same_v<decltype(registry.emplace<int>({})), int &>));
-    ASSERT_TRUE((std::is_same_v<decltype(registry.emplace<empty_type>({})), void>));
+    static_assert((std::is_same_v<decltype(registry.emplace<int>({})), int &>));
+    static_assert((std::is_same_v<decltype(registry.emplace<empty_type>({})), void>));
 
-    ASSERT_TRUE((std::is_same_v<decltype(registry.get<int>({})), int &>));
-    ASSERT_TRUE((std::is_same_v<decltype(registry.get<int, const char>({})), std::tuple<int &, const char &>>));
+    static_assert((std::is_same_v<decltype(registry.get<int>({})), int &>));
+    static_assert((std::is_same_v<decltype(registry.get<int, const char>({})), std::tuple<int &, const char &>>));
 
-    ASSERT_TRUE((std::is_same_v<decltype(registry.try_get<int>({})), int *>));
-    ASSERT_TRUE((std::is_same_v<decltype(registry.try_get<int, const char>({})), std::tuple<int *, const char *>>));
+    static_assert((std::is_same_v<decltype(registry.try_get<int>({})), int *>));
+    static_assert((std::is_same_v<decltype(registry.try_get<int, const char>({})), std::tuple<int *, const char *>>));
 
-    ASSERT_TRUE((std::is_same_v<decltype(std::as_const(registry).get<int>({})), const int &>));
-    ASSERT_TRUE((std::is_same_v<decltype(std::as_const(registry).get<int, const char>({})), std::tuple<const int &, const char &>>));
+    static_assert((std::is_same_v<decltype(registry.ctx<int>()), int &>));
+    static_assert((std::is_same_v<decltype(registry.ctx<const char>()), const char &>));
 
-    ASSERT_TRUE((std::is_same_v<decltype(std::as_const(registry).try_get<int>({})), const int *>));
-    ASSERT_TRUE((std::is_same_v<decltype(std::as_const(registry).try_get<int, const char>({})), std::tuple<const int *, const char *>>));
+    static_assert((std::is_same_v<decltype(registry.try_ctx<int>()), int *>));
+    static_assert((std::is_same_v<decltype(registry.try_ctx<const char>()), const char *>));
+
+    static_assert((std::is_same_v<decltype(std::as_const(registry).get<int>({})), const int &>));
+    static_assert((std::is_same_v<decltype(std::as_const(registry).get<int, const char>({})), std::tuple<const int &, const char &>>));
+
+    static_assert((std::is_same_v<decltype(std::as_const(registry).try_get<int>({})), const int *>));
+    static_assert((std::is_same_v<decltype(std::as_const(registry).try_get<int, const char>({})), std::tuple<const int *, const char *>>));
+
+    static_assert((std::is_same_v<decltype(std::as_const(registry).ctx<int>()), const int &>));
+    static_assert((std::is_same_v<decltype(std::as_const(registry).ctx<const char>()), const char &>));
+
+    static_assert((std::is_same_v<decltype(std::as_const(registry).try_ctx<int>()), const int *>));
+    static_assert((std::is_same_v<decltype(std::as_const(registry).try_ctx<const char>()), const char *>));
 }
 
 TEST(Registry, MoveOnlyComponent) {