Mac: Make ScopedTypeRef require explicit constructors

base/mac/scoped_typeref.h

 // Copyright 2014 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 BASE_MAC_SCOPED_TYPEREF_H_
 #define BASE_MAC_SCOPED_TYPEREF_H_
 
 #include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/memory/scoped_policy.h"
@@ skipping 35 matching lines @@
 // is to |ASSUME|.
 
 template<typename T>
 struct ScopedTypeRefTraits;
 
 template<typename T, typename Traits = ScopedTypeRefTraits<T>>
 class ScopedTypeRef {
  public:
   typedef T element_type;
 
-  ScopedTypeRef(
+  explicit ScopedTypeRef(
       T object = Traits::InvalidValue(),
       base::scoped_policy::OwnershipPolicy policy = base::scoped_policy::ASSUME)
       : object_(object) {
     if (object_ && policy == base::scoped_policy::RETAIN)
       object_ = Traits::Retain(object_);
   }
 
   ScopedTypeRef(const ScopedTypeRef<T, Traits>& that)
       : object_(that.object_) {
     if (object_)
       object_ = Traits::Retain(object_);
   }
 
-  // Without this, passing a ScopedTypeRef<A,TraitsX> to construct a
-  // ScopedTypeRef<A,TraitsY> would automatically cast down to an A, and then
-  // ASSUME ownership of A, when a retain is what was needed.
-  template<typename OtherTraits>
-  ScopedTypeRef(const ScopedTypeRef<T, OtherTraits>& that_with_other_traits)
-      : object_(that_with_other_traits.get()) {
+  // This allows passing an object to a function that takes its superclass.
+  template <typename R, typename RTraits>
+  explicit ScopedTypeRef(const ScopedTypeRef<R, RTraits>& that_as_subclass)
+      : object_(that_as_subclass.get()) {
     if (object_)
       object_ = Traits::Retain(object_);
   }
 
   ScopedTypeRef(ScopedTypeRef<T, Traits>&& that) : object_(that.object_) {
     that.object_ = Traits::InvalidValue();
   }
 
   ~ScopedTypeRef() {
     if (object_)
@@ skipping 54 matching lines @@
     return temp;
   }
 
  private:
   T object_;
 };
 
 }  // namespace base
 
 #endif  // BASE_MAC_SCOPED_TYPEREF_H_