Define illegal zone_allocator constructor only for MSVS

src/zone/zone-allocator.h

 // Copyright 2014 the V8 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.
 
 #ifndef V8_ZONE_ZONE_ALLOCATOR_H_
 #define V8_ZONE_ZONE_ALLOCATOR_H_
 #include <limits>
 
 #include "src/zone/zone.h"
 
 namespace v8 {
 namespace internal {
 
 template <typename T>
 class zone_allocator {
  public:
   typedef T* pointer;
   typedef const T* const_pointer;
   typedef T& reference;
   typedef const T& const_reference;
   typedef T value_type;
   typedef size_t size_type;
   typedef ptrdiff_t difference_type;
   template <class O>
   struct rebind {
     typedef zone_allocator<O> other;
   };
 
-  // TODO(bbudge) Remove when V8 updates to MSVS 2015. See crbug.com/603131.
+#ifdef V8_CC_MSVC
+  // MSVS unfortunately requires the default constructor to be defined.
   zone_allocator() : zone_(nullptr) { UNREACHABLE(); }
+#endif
   explicit zone_allocator(Zone* zone) throw() : zone_(zone) {}
   explicit zone_allocator(const zone_allocator& other) throw()
       : zone_(other.zone_) {}
   template <typename U>
   zone_allocator(const zone_allocator<U>& other) throw() : zone_(other.zone_) {}
   template <typename U>
   friend class zone_allocator;
 
   pointer address(reference x) const { return &x; }
   const_pointer address(const_reference x) const { return &x; }
@@ skipping 30 matching lines @@
  private:
   Zone* zone_;
 };
 
 typedef zone_allocator<bool> ZoneBoolAllocator;
 typedef zone_allocator<int> ZoneIntAllocator;
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ZONE_ZONE_ALLOCATOR_H_