Include <algorithm> if you use functions from <algorithm>.

third_party/WebKit/Source/wtf/Optional.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef Optional_h
 #define Optional_h
 
 #include "wtf/Alignment.h"
 #include "wtf/Assertions.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/StdLibExtras.h"
 #include "wtf/Utility.h"
 
+#include <new>
+
 namespace WTF {
 
 // This is a lightweight template similar to std::experimental::optional.
 // It currently does not support assignment, swapping, comparison, etc.
 //
 // Use this instead of OwnPtr for cases where you only want to conditionally
 // construct a "scope" object.
 //
 // Example:
 //   Optional<DrawingRecorder> recorder;
@@ skipping 34 matching lines @@
 private:
     T* m_ptr;
     AlignedBuffer<sizeof(T), WTF_ALIGN_OF(T)> m_storage;
 };
 
 } // namespace WTF
 
 using WTF::Optional;
 
 #endif // Optional_h