Replace rtc::cricket::Settable with rtc::Maybe

webrtc/base/maybe.h

 /*
  *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 18 matching lines @@
 // assume that it just doesn't contain a value anymore.
 //
 // TODO(kwiberg): Get rid of this class when the standard library has
 // std::optional (and we're allowed to use it).
 template <typename T>
 class Maybe final {
  public:
   // Construct an empty Maybe.
   Maybe() : has_value_(false) {}
 
-  // Construct a Maybe that contains a value. Note: These are non-explicit, so
-  // that a T will implicitly convert to Maybe<T>.
-  Maybe(const T& val) : value_(val), has_value_(true) {}
-  Maybe(T&& val) : value_(static_cast<T&&>(val)), has_value_(true) {}
+  // Construct a Maybe that contains a value.
+  explicit Maybe(const T& val) : value_(val), has_value_(true) {}
+  explicit Maybe(T&& val) : value_(static_cast<T&&>(val)), has_value_(true) {}
 
   // Copy and move constructors.
   // TODO(kwiberg): =default the move constructor when MSVC supports it.
   Maybe(const Maybe&) = default;
   Maybe(Maybe&& m)
       : value_(static_cast<T&&>(m.value_)), has_value_(m.has_value_) {}
 
-  // Assignment. Note that we allow assignment from either Maybe<T> or plain T.
+  // Assignment.
   // TODO(kwiberg): =default the move assignment op when MSVC supports it.
   Maybe& operator=(const Maybe&) = default;
   Maybe& operator=(Maybe&& m) {
     value_ = static_cast<T&&>(m.value_);
     has_value_ = m.has_value_;
     return *this;
   }
-  Maybe& operator=(const T& val) {
-    value_ = val;
-    has_value_ = true;
-    return *this;
-  }
-  Maybe& operator=(T&& val) {
-    value_ = static_cast<T&&>(val);
-    has_value_ = true;
-    return *this;
-  }
 
   friend void swap(Maybe& m1, Maybe& m2) {
     using std::swap;
     swap(m1.value_, m2.value_);
     swap(m1.has_value_, m2.has_value_);
   }
 
   // Conversion to bool to test if we have a value.
   explicit operator bool() const { return has_value_; }
 
@@ skipping 34 matching lines @@
  private:
   // Invariant: Unless *this has been moved from, value_ is default-initialized
   // (or copied or moved from a default-initialized T) if !has_value_.
   T value_;
   bool has_value_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_MAYBE_H_