diff --git a/absl/flags/marshalling.h b/absl/flags/marshalling.h
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+//
+//  Copyright 2019 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.
+//
+// -----------------------------------------------------------------------------
+// File: marshalling.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the API for extending Abseil flag support to
+// custom types, and defines the set of overloads for fundamental types.
+//
+// Out of the box, the Abseil flags library supports the following types:
+//
+// * `bool`
+// * `int16_t`
+// * `uint16_t`
+// * `int32_t`
+// * `uint32_t`
+// * `int64_t`
+// * `uint64_t`
+// * `float`
+// * `double`
+// * `std::string`
+// * `std::vector<std::string>`
+// * `absl::LogSeverity` (provided here due to dependency ordering)
+//
+// Note that support for integral types is implemented using overloads for
+// variable-width fundamental types (`short`, `int`, `long`, etc.). However,
+// you should prefer the fixed-width integral types (`int32_t`, `uint64_t`,
+// etc.) we've noted above within flag definitions.
+
+//
+// In addition, several Abseil libraries provide their own custom support for
+// Abseil flags.
+//
+// The Abseil time library provides the following support for civil time values:
+//
+// * `absl::CivilSecond`
+// * `absl::CivilMinute`
+// * `absl::CivilHour`
+// * `absl::CivilDay`
+// * `absl::CivilMonth`
+// * `absl::CivilYear`
+//
+// and also provides support for the following absolute time values:
+//
+// * `absl::Duration`
+// * `absl::Time`
+//
+// Additional support for Abseil types will be noted here as it is added.
+//
+// You can also provide your own custom flags by adding overloads for
+// `AbslParseFlag()` and `AbslUnparseFlag()` to your type definitions. (See
+// below.)
+//
+// -----------------------------------------------------------------------------
+// Adding Type Support for Abseil Flags
+// -----------------------------------------------------------------------------
+//
+// To add support for your user-defined type, add overloads of `AbslParseFlag()`
+// and `AbslUnparseFlag()` as free (non-member) functions to your type. If `T`
+// is a class type, these functions can be friend function definitions. These
+// overloads must be added to the same namespace where the type is defined, so
+// that they can be discovered by Argument-Dependent Lookup (ADL).
+//
+// Example:
+//
+//   namespace foo {
+//
+//   enum OutputMode { kPlainText, kHtml };
+//
+//   // AbslParseFlag converts from a string to OutputMode.
+//   // Must be in same namespace as OutputMode.
+//
+//   // Parses an OutputMode from the command line flag value `text. Returns
+//   // `true` and sets `*mode` on success; returns `false` and sets `*error`
+//   // on failure.
+//   bool AbslParseFlag(absl::string_view text,
+//                      OutputMode* mode,
+//                      std::string* error) {
+//     if (text == "plaintext") {
+//       *mode = kPlainText;
+//       return true;
+//     }
+//     if (text == "html") {
+//       *mode = kHtml;
+//      return true;
+//     }
+//     *error = "unknown value for enumeration";
+//     return false;
+//  }
+//
+//  // AbslUnparseFlag converts from an OutputMode to a string.
+//  // Must be in same namespace as OutputMode.
+//
+//  // Returns a textual flag value corresponding to the OutputMode `mode`.
+//  std::string AbslUnparseFlag(OutputMode mode) {
+//    switch (mode) {
+//      case kPlainText: return "plaintext";
+//      case kHtml: return "html";
+//    }
+//    return absl::StrCat(mode);
+//  }
+//
+// Notice that neither `AbslParseFlag()` nor `AbslUnparseFlag()` are class
+// members, but free functions. `AbslParseFlag/AbslUnparseFlag()` overloads
+// for a type should only be declared in the same file and namespace as said
+// type. The proper `AbslParseFlag/AbslUnparseFlag()` implementations for a
+// given type will be discovered via Argument-Dependent Lookup (ADL).
+//
+// `AbslParseFlag()` may need, in turn, to parse simpler constituent types
+// using `absl::ParseFlag()`. For example, a custom struct `MyFlagType`
+// consisting of a `std::pair<int, std::string>` would add an `AbslParseFlag()`
+// overload for its `MyFlagType` like so:
+//
+// Example:
+//
+//   namespace my_flag_type {
+//
+//   struct MyFlagType {
+//     std::pair<int, std::string> my_flag_data;
+//   };
+//
+//   bool AbslParseFlag(absl::string_view text, MyFlagType* flag,
+//                      std::string* err);
+//
+//   std::string AbslUnparseFlag(const MyFlagType&);
+//
+//   // Within the implementation, `AbslParseFlag()` will, in turn invoke
+//   // `absl::ParseFlag()` on its constituent `int` and `std::string` types
+//   // (which have built-in Abseil flag support.
+//
+//   bool AbslParseFlag(absl::string_view text, MyFlagType* flag,
+//                      std::string* err) {
+//     std::pair<absl::string_view, absl::string_view> tokens =
+//         absl::StrSplit(text, ',');
+//     if (!absl::ParseFlag(tokens.first, &flag->my_flag_data.first, err))
+//         return false;
+//     if (!absl::ParseFlag(tokens.second, &flag->my_flag_data.second, err))
+//         return false;
+//     return true;
+//   }
+//
+//   // Similarly, for unparsing, we can simply invoke `absl::UnparseFlag()` on
+//   // the constituent types.
+//   std::string AbslUnparseFlag(const MyFlagType& flag) {
+//     return absl::StrCat(absl::UnparseFlag(flag.my_flag_data.first),
+//                         ",",
+//                         absl::UnparseFlag(flag.my_flag_data.second));
+//   }
+#ifndef ABSL_FLAGS_MARSHALLING_H_
+#define ABSL_FLAGS_MARSHALLING_H_
+
+#include <string>
+#include <vector>
+
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace flags_internal {
+
+// Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types.
+bool AbslParseFlag(absl::string_view, bool*, std::string*);
+bool AbslParseFlag(absl::string_view, short*, std::string*);           // NOLINT
+bool AbslParseFlag(absl::string_view, unsigned short*, std::string*);  // NOLINT
+bool AbslParseFlag(absl::string_view, int*, std::string*);             // NOLINT
+bool AbslParseFlag(absl::string_view, unsigned int*, std::string*);    // NOLINT
+bool AbslParseFlag(absl::string_view, long*, std::string*);            // NOLINT
+bool AbslParseFlag(absl::string_view, unsigned long*, std::string*);   // NOLINT
+bool AbslParseFlag(absl::string_view, long long*, std::string*);       // NOLINT
+bool AbslParseFlag(absl::string_view, unsigned long long*,             // NOLINT
+                   std::string*);
+bool AbslParseFlag(absl::string_view, float*, std::string*);
+bool AbslParseFlag(absl::string_view, double*, std::string*);
+bool AbslParseFlag(absl::string_view, std::string*, std::string*);
+bool AbslParseFlag(absl::string_view, std::vector<std::string>*, std::string*);
+
+template <typename T>
+bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) {
+  // Comment on next line provides a good compiler error message if T
+  // does not have AbslParseFlag(absl::string_view, T*, std::string*).
+  return AbslParseFlag(input, dst, err);  // Is T missing AbslParseFlag?
+}
+
+// Strings and std:: containers do not have the same overload resolution
+// considerations as fundamental types. Naming these 'AbslUnparseFlag' means we
+// can avoid the need for additional specializations of Unparse (below).
+std::string AbslUnparseFlag(absl::string_view v);
+std::string AbslUnparseFlag(const std::vector<std::string>&);
+
+template <typename T>
+std::string Unparse(const T& v) {
+  // Comment on next line provides a good compiler error message if T does not
+  // have UnparseFlag.
+  return AbslUnparseFlag(v);  // Is T missing AbslUnparseFlag?
+}
+
+// Overloads for builtin types.
+std::string Unparse(bool v);
+std::string Unparse(short v);               // NOLINT
+std::string Unparse(unsigned short v);      // NOLINT
+std::string Unparse(int v);                 // NOLINT
+std::string Unparse(unsigned int v);        // NOLINT
+std::string Unparse(long v);                // NOLINT
+std::string Unparse(unsigned long v);       // NOLINT
+std::string Unparse(long long v);           // NOLINT
+std::string Unparse(unsigned long long v);  // NOLINT
+std::string Unparse(float v);
+std::string Unparse(double v);
+
+}  // namespace flags_internal
+
+// ParseFlag()
+//
+// Parses a string value into a flag value of type `T`. Do not add overloads of
+// this function for your type directly; instead, add an `AbslParseFlag()`
+// free function as documented above.
+//
+// Some implementations of `AbslParseFlag()` for types which consist of other,
+// constituent types which already have Abseil flag support, may need to call
+// `absl::ParseFlag()` on those consituent string values. (See above.)
+template <typename T>
+inline bool ParseFlag(absl::string_view input, T* dst, std::string* error) {
+  return flags_internal::InvokeParseFlag(input, dst, error);
+}
+
+// UnparseFlag()
+//
+// Unparses a flag value of type `T` into a string value. Do not add overloads
+// of this function for your type directly; instead, add an `AbslUnparseFlag()`
+// free function as documented above.
+//
+// Some implementations of `AbslUnparseFlag()` for types which consist of other,
+// constituent types which already have Abseil flag support, may want to call
+// `absl::UnparseFlag()` on those constituent types. (See above.)
+template <typename T>
+inline std::string UnparseFlag(const T& v) {
+  return flags_internal::Unparse(v);
+}
+
+// Overloads for `absl::LogSeverity` can't (easily) appear alongside that type's
+// definition because it is layered below flags.  See proper documentation in
+// base/log_severity.h.
+enum class LogSeverity : int;
+bool AbslParseFlag(absl::string_view, absl::LogSeverity*, std::string*);
+std::string AbslUnparseFlag(absl::LogSeverity);
+
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_MARSHALLING_H_