Castable: Optimize Switch()

Use a bloom-filter-style early rejection to eliminate whole blocks of
case statements from the switch type checks. Much like IsAnyOf(), the
list of types considered are recursively tested as a whole and
then binary-chopped if there's a potential match, until we test the
individual switch case types.

Bug: tint:1383
Change-Id: I5b30f19ea070e8352bf6b9363f133da906013182
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78544
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

src/castable.h

@@ -465,6 +465,131 @@ class Castable : public BASE {
 /// ```
 struct Default {};
 
+namespace detail {
+
+/// Evaluates to the Switch case type being matched by the switch case function
+/// `FN`.
+/// @note does not handle the Default case
+/// @see Switch().
+template <typename FN>
+using SwitchCaseType = std::remove_const_t<std::remove_pointer_t<
+    traits::ParameterType<std::remove_reference_t<FN>, 0>>>;
+
+/// Evaluates to true if the function `FN` has the signature of a Default case
+/// in a Switch().
+/// @see Switch().
+template <typename FN>
+inline constexpr bool IsDefaultCase =
+    std::is_same_v<traits::ParameterType<std::remove_reference_t<FN>, 0>,
+                   Default>;
+
+/// Searches the list of Switch cases for a Default case, returning the index of
+/// the Default case. If the a Default case is not found in the tuple, then -1
+/// is returned.
+template <typename TUPLE, std::size_t START_IDX = 0>
+constexpr int IndexOfDefaultCase() {
+  if constexpr (START_IDX < std::tuple_size_v<TUPLE>) {
+    return IsDefaultCase<std::tuple_element_t<START_IDX, TUPLE>>
+               ? static_cast<int>(START_IDX)
+               : IndexOfDefaultCase<TUPLE, START_IDX + 1>();
+  } else {
+    return -1;
+  }
+}
+
+/// The implementation of Switch() for non-Default cases.
+/// Switch splits the cases into two a low and high block of cases, and quickly
+/// rules out blocks that cannot match by comparing the TypeInfo::HashCode of
+/// the object and the cases in the block. If a block of cases may match the
+/// given object's type, then that block is split into two, and the process
+/// recurses. When NonDefaultCases() is called with a single case, then As<>
+/// will be used to dynamically cast to the case type and if the cast succeeds,
+/// then the case handler is called.
+/// @returns true if a case handler was found, otherwise false.
+template <typename T, typename RETURN_TYPE, typename... CASES>
+inline bool NonDefaultCases(T* object,
+                            const TypeInfo* type,
+                            RETURN_TYPE* result,
+                            std::tuple<CASES...>&& cases) {
+  using Cases = std::tuple<CASES...>;
+
+  (void)result;  // Not always used, avoid warning.
+
+  static constexpr bool kHasReturnType = !std::is_same_v<RETURN_TYPE, void>;
+  static constexpr size_t kNumCases = sizeof...(CASES);
+
+  if constexpr (kNumCases == 0) {
+    // No cases. Nothing to do.
+    return false;
+  } else if constexpr (kNumCases == 1) {  // NOLINT: cpplint doesn't understand
+                                          // `else if constexpr`
+    // Single case.
+    using CaseFunc = std::tuple_element_t<0, Cases>;
+    static_assert(!IsDefaultCase<CaseFunc>,
+                  "NonDefaultCases called with a Default case");
+    // Attempt to dynamically cast the object to the handler type. If that
+    // succeeds, call the case handler with the cast object.
+    using CaseType = SwitchCaseType<CaseFunc>;
+    if (auto* ptr = As<CaseType>(object)) {
+      if constexpr (kHasReturnType) {
+        *result = std::get<0>(cases)(ptr);
+      } else {
+        std::get<0>(cases)(ptr);
+      }
+      return true;
+    }
+    return false;
+  } else {
+    // Multiple cases.
+    // Check the hashcode bits to see if there's any possibility of a case
+    // matching in these cases. If there isn't, we can skip all these cases.
+    if (type->full_hashcode &
+        TypeInfo::CombinedHashCodeOf<SwitchCaseType<CASES>...>()) {
+      // There's a possibility. We need to scan further.
+      // Split the cases into two, and recurse.
+      constexpr size_t kMid = kNumCases / 2;
+      return NonDefaultCases(object, type, result,
+                             traits::Slice<0, kMid>(cases)) ||
+             NonDefaultCases(object, type, result,
+                             traits::Slice<kMid, kNumCases - kMid>(cases));
+    } else {
+      return false;
+    }
+  }
+}
+
+/// The implementation of Switch() for all cases.
+/// @see NonDefaultCases
+template <typename T, typename RETURN_TYPE, typename... CASES>
+inline void SwitchCases(T* object,
+                        const TypeInfo* type,
+                        RETURN_TYPE* result,
+                        std::tuple<CASES...>&& cases) {
+  using Cases = std::tuple<CASES...>;
+  static constexpr int kDefaultIndex = detail::IndexOfDefaultCase<Cases>();
+  static_assert(kDefaultIndex == -1 || std::tuple_size_v<Cases> - 1,
+                "Default case must be last in Switch()");
+  static constexpr bool kHasDefaultCase = kDefaultIndex >= 0;
+  static constexpr bool kHasReturnType = !std::is_same_v<RETURN_TYPE, void>;
+
+  if constexpr (kHasDefaultCase) {
+    // Evaluate non-default cases.
+    if (!detail::NonDefaultCases<T>(object, type, result,
+                                    traits::Slice<0, kDefaultIndex>(cases))) {
+      // Nothing matched. Evaluate default case.
+      if constexpr (kHasReturnType) {
+        *result = std::get<kDefaultIndex>(cases)({});
+      } else {
+        std::get<kDefaultIndex>(cases)({});
+      }
+    }
+  } else {
+    detail::NonDefaultCases<T>(object, type, result, std::move(cases));
+  }
+}
+
+}  // namespace detail
+
 /// Switch is used to dispatch one of the provided callback case handler
 /// functions based on the type of `object` and the parameter type of the case
 /// handlers. Switch will sequentially check the type of `object` against each
@@ -493,62 +618,26 @@ struct Default {};
 /// ```
 ///
 /// @param object the object who's type is used to
-/// @param first_case the first switch case
+/// @param cases the switch cases
-/// @param other_cases additional switch cases (optional)
 /// @return the value returned by the called case. If no cases matched, then the
 /// zero value for the consistent case type.
-template <typename T, typename FIRST_CASE, typename... OTHER_CASES>
+template <typename T, typename... CASES>
-traits::ReturnType<FIRST_CASE>  //
+inline auto Switch(T* object, CASES&&... cases) {
-Switch(T* object, FIRST_CASE&& first_case, OTHER_CASES&&... other_cases) {
+  using Cases = std::tuple<CASES...>;
-  using ReturnType = traits::ReturnType<FIRST_CASE>;
+  using ReturnType = traits::ReturnType<std::tuple_element_t<0, Cases>>;
-  using CaseType = std::remove_pointer_t<traits::ParameterType<FIRST_CASE, 0>>;
   static constexpr bool kHasReturnType = !std::is_same_v<ReturnType, void>;
-  static_assert(traits::SignatureOfT<FIRST_CASE>::parameter_count == 1,
+
-                "Switch case must have a single parameter");
+  auto& type = object->TypeInfo();
-  if constexpr (std::is_same_v<CaseType, Default>) {
+
-    // Default case. Must be last.
+  if constexpr (kHasReturnType) {
-    (void)object;  // 'object' is not used by the Default case.
+    ReturnType res = {};
-    static_assert(sizeof...(other_cases) == 0,
+    detail::SwitchCases(object, &type, &res,
-                  "Switch Default case must come last");
+                        std::forward_as_tuple(std::forward<CASES>(cases)...));
-    if constexpr (kHasReturnType) {
+    return res;
-      return first_case({});
-    } else {
-      first_case({});
-      return;
-    }
   } else {
-    // Regular case.
+    detail::SwitchCases<T, void>(
-    static_assert(traits::IsTypeOrDerived<CaseType, CastableBase>::value,
+        object, &type, nullptr,
-                  "Switch case parameter is not a Castable pointer");
+        std::forward_as_tuple(std::forward<CASES>(cases)...));
-    // Does the case match?
-    if (auto* ptr = As<CaseType>(object)) {
-      if constexpr (kHasReturnType) {
-        return first_case(ptr);
-      } else {
-        first_case(ptr);
-        return;
-      }
-    }
-    // Case did not match. Got any more cases to try?
-    if constexpr (sizeof...(other_cases) > 0) {
-      // Try the next cases...
-      if constexpr (kHasReturnType) {
-        auto res = Switch(object, std::forward<OTHER_CASES>(other_cases)...);
-        static_assert(std::is_same_v<decltype(res), ReturnType>,
-                      "Switch case types do not have consistent return type");
-        return res;
-      } else {
-        Switch(object, std::forward<OTHER_CASES>(other_cases)...);
-        return;
-      }
-    } else {
-      // That was the last case. No cases matched.
-      if constexpr (kHasReturnType) {
-        return {};
-      } else {
-        return;
-      }
-    }
   }
 }
 

src/traits.h

@@ -124,16 +124,20 @@ constexpr auto Range() {
 namespace detail {
 
 /// @returns the tuple `t` swizzled by `INDICES`
-template <class TUPLE, std::size_t... INDICES>
+template <typename TUPLE, std::size_t... INDICES>
-constexpr auto Swizzle(TUPLE&& t, std::index_sequence<INDICES...>) {
+constexpr auto Swizzle(TUPLE&& t, std::index_sequence<INDICES...>)
-  return std::make_tuple(std::get<INDICES>(std::forward<TUPLE>(t))...);
+    -> std::tuple<
+        std::tuple_element_t<INDICES, std::remove_reference_t<TUPLE>>...> {
+  return {std::forward<
+      std::tuple_element_t<INDICES, std::remove_reference_t<TUPLE>>>(
+      std::get<INDICES>(std::forward<TUPLE>(t)))...};
 }
 
 /// @returns a nullptr of the tuple type `TUPLE` swizzled by `INDICES`.
 /// @note: This function is intended to be used in a `decltype()` expression,
 /// and returns a pointer-to-tuple as the tuple may hold non-constructable
 /// types.
-template <class TUPLE, std::size_t... INDICES>
+template <typename TUPLE, std::size_t... INDICES>
 constexpr auto* SwizzlePtrTy(std::index_sequence<INDICES...>) {
   using Swizzled = std::tuple<std::tuple_element_t<INDICES, TUPLE>...>;
   return static_cast<Swizzled*>(nullptr);

src/traits_test.cc

@@ -28,14 +28,14 @@ void F3(int, S, float) {}
 TEST(ParamType, Function) {
   F1({});        // Avoid unused method warning
   F3(0, {}, 0);  // Avoid unused method warning
-  static_assert(std::is_same_v<ParameterType<decltype(&F1), 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&F1), 0>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&F3), 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&F3), 0>, int>);
-  static_assert(std::is_same_v<ParameterType<decltype(&F3), 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&F3), 1>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&F3), 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&F3), 2>, float>);
-  static_assert(std::is_same_v<ReturnType<decltype(&F1)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&F1)>, void>);
-  static_assert(std::is_same_v<ReturnType<decltype(&F3)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&F3)>, void>);
-  static_assert(SignatureOfT<decltype(&F1)>::parameter_count == 1, "");
+  static_assert(SignatureOfT<decltype(&F1)>::parameter_count == 1);
-  static_assert(SignatureOfT<decltype(&F3)>::parameter_count == 3, "");
+  static_assert(SignatureOfT<decltype(&F3)>::parameter_count == 3);
 }
 
 TEST(ParamType, Method) {
@@ -46,14 +46,14 @@ TEST(ParamType, Method) {
   };
   C().F1({});        // Avoid unused method warning
   C().F3(0, {}, 0);  // Avoid unused method warning
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>);
-  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1, "");
+  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1);
-  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3, "");
+  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3);
 }
 
 TEST(ParamType, ConstMethod) {
@@ -64,14 +64,14 @@ TEST(ParamType, ConstMethod) {
   };
   C().F1({});        // Avoid unused method warning
   C().F3(0, {}, 0);  // Avoid unused method warning
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>);
-  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1, "");
+  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1);
-  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3, "");
+  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3);
 }
 
 TEST(ParamType, StaticMethod) {
@@ -82,55 +82,55 @@ TEST(ParamType, StaticMethod) {
   };
   C::F1({});        // Avoid unused method warning
   C::F3(0, {}, 0);  // Avoid unused method warning
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F1), 0>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 0>, int>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 1>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(&C::F3), 2>, float>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F1)>, void>);
-  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(&C::F3)>, void>);
-  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1, "");
+  static_assert(SignatureOfT<decltype(&C::F1)>::parameter_count == 1);
-  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3, "");
+  static_assert(SignatureOfT<decltype(&C::F3)>::parameter_count == 3);
 }
 
 TEST(ParamType, FunctionLike) {
   using F1 = std::function<void(S)>;
   using F3 = std::function<void(int, S, float)>;
-  static_assert(std::is_same_v<ParameterType<F1, 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<F1, 0>, S>);
-  static_assert(std::is_same_v<ParameterType<F3, 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<F3, 0>, int>);
-  static_assert(std::is_same_v<ParameterType<F3, 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<F3, 1>, S>);
-  static_assert(std::is_same_v<ParameterType<F3, 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<F3, 2>, float>);
-  static_assert(std::is_same_v<ReturnType<F1>, void>, "");
+  static_assert(std::is_same_v<ReturnType<F1>, void>);
-  static_assert(std::is_same_v<ReturnType<F3>, void>, "");
+  static_assert(std::is_same_v<ReturnType<F3>, void>);
-  static_assert(SignatureOfT<F1>::parameter_count == 1, "");
+  static_assert(SignatureOfT<F1>::parameter_count == 1);
-  static_assert(SignatureOfT<F3>::parameter_count == 3, "");
+  static_assert(SignatureOfT<F3>::parameter_count == 3);
 }
 
 TEST(ParamType, Lambda) {
   auto l1 = [](S) {};
   auto l3 = [](int, S, float) {};
-  static_assert(std::is_same_v<ParameterType<decltype(l1), 0>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(l1), 0>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(l3), 0>, int>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(l3), 0>, int>);
-  static_assert(std::is_same_v<ParameterType<decltype(l3), 1>, S>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(l3), 1>, S>);
-  static_assert(std::is_same_v<ParameterType<decltype(l3), 2>, float>, "");
+  static_assert(std::is_same_v<ParameterType<decltype(l3), 2>, float>);
-  static_assert(std::is_same_v<ReturnType<decltype(l1)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(l1)>, void>);
-  static_assert(std::is_same_v<ReturnType<decltype(l3)>, void>, "");
+  static_assert(std::is_same_v<ReturnType<decltype(l3)>, void>);
-  static_assert(SignatureOfT<decltype(l1)>::parameter_count == 1, "");
+  static_assert(SignatureOfT<decltype(l1)>::parameter_count == 1);
-  static_assert(SignatureOfT<decltype(l3)>::parameter_count == 3, "");
+  static_assert(SignatureOfT<decltype(l3)>::parameter_count == 3);
 }
 
 TEST(Slice, Empty) {
   auto sliced = Slice<0, 0>(std::make_tuple<>());
-  static_assert(std::tuple_size_v<decltype(sliced)> == 0, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 0);
 }
 
 TEST(Slice, SingleElementSliceEmpty) {
   auto sliced = Slice<0, 0>(std::make_tuple<int>(1));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 0, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 0);
 }
 
 TEST(Slice, SingleElementSliceFull) {
   auto sliced = Slice<0, 1>(std::make_tuple<int>(1));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 1, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 1);
   static_assert(std::is_same_v<std::tuple_element_t<0, decltype(sliced)>, int>,
                 "");
   EXPECT_EQ(std::get<0>(sliced), 1);
@@ -138,18 +138,18 @@ TEST(Slice, SingleElementSliceFull) {
 
 TEST(Slice, MixedTupleSliceEmpty) {
   auto sliced = Slice<1, 0>(std::make_tuple<int, bool, float>(1, true, 2.0f));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 0, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 0);
 }
 
 TEST(Slice, MixedTupleSliceFull) {
   auto sliced = Slice<0, 3>(std::make_tuple<int, bool, float>(1, true, 2.0f));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 3, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 3);
   static_assert(std::is_same_v<std::tuple_element_t<0, decltype(sliced)>, int>,
                 "");
   static_assert(std::is_same_v<std::tuple_element_t<1, decltype(sliced)>, bool>,
                 "");
   static_assert(
-      std::is_same_v<std::tuple_element_t<2, decltype(sliced)>, float>, "");
+      std::is_same_v<std::tuple_element_t<2, decltype(sliced)>, float>);
   EXPECT_EQ(std::get<0>(sliced), 1);
   EXPECT_EQ(std::get<1>(sliced), true);
   EXPECT_EQ(std::get<2>(sliced), 2.0f);
@@ -157,7 +157,7 @@ TEST(Slice, MixedTupleSliceFull) {
 
 TEST(Slice, MixedTupleSliceLowPart) {
   auto sliced = Slice<0, 2>(std::make_tuple<int, bool, float>(1, true, 2.0f));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 2, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 2);
   static_assert(std::is_same_v<std::tuple_element_t<0, decltype(sliced)>, int>,
                 "");
   static_assert(std::is_same_v<std::tuple_element_t<1, decltype(sliced)>, bool>,
@@ -168,56 +168,67 @@ TEST(Slice, MixedTupleSliceLowPart) {
 
 TEST(Slice, MixedTupleSliceHighPart) {
   auto sliced = Slice<1, 2>(std::make_tuple<int, bool, float>(1, true, 2.0f));
-  static_assert(std::tuple_size_v<decltype(sliced)> == 2, "");
+  static_assert(std::tuple_size_v<decltype(sliced)> == 2);
   static_assert(std::is_same_v<std::tuple_element_t<0, decltype(sliced)>, bool>,
                 "");
   static_assert(
-      std::is_same_v<std::tuple_element_t<1, decltype(sliced)>, float>, "");
+      std::is_same_v<std::tuple_element_t<1, decltype(sliced)>, float>);
   EXPECT_EQ(std::get<0>(sliced), true);
   EXPECT_EQ(std::get<1>(sliced), 2.0f);
 }
 
+TEST(Slice, PreservesRValueRef) {
+  int i;
+  int& int_ref = i;
+  auto tuple = std::forward_as_tuple(std::move(int_ref));
+  static_assert(std::is_same_v<int&&,  //
+                               std::tuple_element_t<0, decltype(tuple)>>);
+  auto sliced = Slice<0, 1>(std::move(tuple));
+  static_assert(std::is_same_v<int&&,  //
+                               std::tuple_element_t<0, decltype(sliced)>>);
+}
+
 TEST(SliceTuple, Empty) {
   using sliced = SliceTuple<0, 0, std::tuple<>>;
-  static_assert(std::tuple_size_v<sliced> == 0, "");
+  static_assert(std::tuple_size_v<sliced> == 0);
 }
 
 TEST(SliceTuple, SingleElementSliceEmpty) {
   using sliced = SliceTuple<0, 0, std::tuple<int>>;
-  static_assert(std::tuple_size_v<sliced> == 0, "");
+  static_assert(std::tuple_size_v<sliced> == 0);
 }
 
 TEST(SliceTuple, SingleElementSliceFull) {
   using sliced = SliceTuple<0, 1, std::tuple<int>>;
-  static_assert(std::tuple_size_v<sliced> == 1, "");
+  static_assert(std::tuple_size_v<sliced> == 1);
-  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>);
 }
 
 TEST(SliceTuple, MixedTupleSliceEmpty) {
   using sliced = SliceTuple<1, 0, std::tuple<int, bool, float>>;
-  static_assert(std::tuple_size_v<sliced> == 0, "");
+  static_assert(std::tuple_size_v<sliced> == 0);
 }
 
 TEST(SliceTuple, MixedTupleSliceFull) {
   using sliced = SliceTuple<0, 3, std::tuple<int, bool, float>>;
-  static_assert(std::tuple_size_v<sliced> == 3, "");
+  static_assert(std::tuple_size_v<sliced> == 3);
-  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>);
-  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, bool>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, bool>);
-  static_assert(std::is_same_v<std::tuple_element_t<2, sliced>, float>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<2, sliced>, float>);
 }
 
 TEST(SliceTuple, MixedTupleSliceLowPart) {
   using sliced = SliceTuple<0, 2, std::tuple<int, bool, float>>;
-  static_assert(std::tuple_size_v<sliced> == 2, "");
+  static_assert(std::tuple_size_v<sliced> == 2);
-  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, int>);
-  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, bool>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, bool>);
 }
 
 TEST(SliceTuple, MixedTupleSliceHighPart) {
   using sliced = SliceTuple<1, 2, std::tuple<int, bool, float>>;
-  static_assert(std::tuple_size_v<sliced> == 2, "");
+  static_assert(std::tuple_size_v<sliced> == 2);
-  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, bool>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<0, sliced>, bool>);
-  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, float>, "");
+  static_assert(std::is_same_v<std::tuple_element_t<1, sliced>, float>);
 }
 
 }  // namespace traits