merge(3p/absl): subtree merge of Abseil up to e19260f

... notably, this includes Abseil's own StatusOr type, which
conflicted with our implementation (that was taken from TensorFlow).

Change-Id: Ie7d6764b64055caaeb8dc7b6b9d066291e6b538f

third_party/abseil_cpp/absl/strings/internal/string_constant.h

@@ -0,0 +1,70 @@
+// Copyright 2020 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
+//
+//      https://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_STRINGS_INTERNAL_STRING_CONSTANT_H_
+#define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_
+
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+// StringConstant<T> represents a compile time string constant.
+// It can be accessed via its `absl::string_view value` static member.
+// It is guaranteed that the `string_view` returned has constant `.data()`,
+// constant `.size()` and constant `value[i]` for all `0 <= i < .size()`
+//
+// The `T` is an opaque type. It is guaranteed that different string constants
+// will have different values of `T`. This allows users to associate the string
+// constant with other static state at compile time.
+//
+// Instances should be made using the `MakeStringConstant()` factory function
+// below.
+template <typename T>
+struct StringConstant {
+ private:
+  // Returns true if `view` points to constant data.
+  // Otherwise, it can't be constant evaluated.
+  static constexpr bool ValidateConstant(absl::string_view view) {
+    return view.empty() || 2 * view[0] != 1;
+  }
+
+ public:
+  static constexpr absl::string_view value = T{}();
+  constexpr absl::string_view operator()() const { return value; }
+
+  static_assert(ValidateConstant(value),
+                "The input string_view must point to constant data.");
+};
+
+template <typename T>
+constexpr absl::string_view StringConstant<T>::value;  // NOLINT
+
+// Factory function for `StringConstant` instances.
+// It supports callables that have a constexpr default constructor and a
+// constexpr operator().
+// It must return an `absl::string_view` or `const char*` pointing to constant
+// data. This is validated at compile time.
+template <typename T>
+constexpr StringConstant<T> MakeStringConstant(T) {
+  return {};
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_