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diff --git a/absl/container/internal/common.h b/absl/container/internal/common.h
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+// Copyright 2018 The Abseil 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 ABSL_CONTAINER_INTERNAL_CONTAINER_H_
+#define ABSL_CONTAINER_INTERNAL_CONTAINER_H_
+
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+namespace container_internal {
+
+template <class, class = void>
+struct IsTransparent : std::false_type {};
+template <class T>
+struct IsTransparent<T, absl::void_t<typename T::is_transparent>>
+    : std::true_type {};
+
+template <bool is_transparent>
+struct KeyArg {
+  // Transparent. Forward `K`.
+  template <typename K, typename key_type>
+  using type = K;
+};
+
+template <>
+struct KeyArg<false> {
+  // Not transparent. Always use `key_type`.
+  template <typename K, typename key_type>
+  using type = key_type;
+};
+
+}  // namespace container_internal
+}   // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_H_