Add TypeTraits.h for Dawn's additional type traits.

Adds the first type trait that checks that the equality operator is
defined for two types.

Bug: dawn:441
Change-Id: Ied80c5d876739272c07d513727a3ee709a721eab
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/35522
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/common/BUILD.gn

@@ -186,6 +186,7 @@ if (is_win || is_linux || is_chromeos || is_mac || is_fuchsia || is_android) {
       "SwapChainUtils.h",
       "SystemUtils.cpp",
       "SystemUtils.h",
+      "TypeTraits.h",
       "TypedInteger.h",
       "UnderlyingType.h",
       "ityp_array.h",

src/common/CMakeLists.txt

@@ -48,6 +48,7 @@ target_sources(dawn_common PRIVATE
     "SwapChainUtils.h"
     "SystemUtils.cpp"
     "SystemUtils.h"
+    "TypeTraits.h"
     "TypedInteger.h"
     "UnderlyingType.h"
     "ityp_array.h"

src/common/TypeTraits.h

@@ -0,0 +1,34 @@
+// Copyright 2020 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef COMMON_TYPETRAITS_H_
+#define COMMON_TYPETRAITS_H_
+
+#include <type_traits>
+
+template <typename LHS, typename RHS = LHS, typename T = void>
+struct HasEqualityOperator {
+    static constexpr const bool value = false;
+};
+
+template <typename LHS, typename RHS>
+struct HasEqualityOperator<
+    LHS,
+    RHS,
+    std::enable_if_t<
+        std::is_same<decltype(std::declval<LHS>() == std::declval<RHS>()), bool>::value>> {
+    static constexpr const bool value = true;
+};
+
+#endif  // COMMON_TYPE_TRAITS_H_

src/dawn_native/SubresourceStorage.h

@@ -16,6 +16,7 @@
 #define DAWNNATIVE_SUBRESOURCESTORAGE_H_
 
 #include "common/Assert.h"
+#include "common/TypeTraits.h"
 #include "dawn_native/EnumMaskIterator.h"
 #include "dawn_native/Subresource.h"
 
@@ -105,6 +106,9 @@ namespace dawn_native {
     template <typename T>
     class SubresourceStorage {
       public:
+        static_assert(std::is_copy_assignable<T>::value, "T must be copyable");
+        static_assert(HasEqualityOperator<T>::value, "T requires bool operator == (T, T)");
+
         // Creates the storage with the given "dimensions" and all subresources starting with the
         // initial value.
         SubresourceStorage(Aspect aspects,