[iOS/OS X] Allow base::scoped_nsobject<> to be used when ARC is enabled.

base/mac/scoped_nsobject.h

Index: base/mac/scoped_nsobject.h
diff --git a/base/mac/scoped_nsobject.h b/base/mac/scoped_nsobject.h
index 04c5877db718e5926584fda5c07eeede5f7078c2..e4fc5c4b3ed5845c18478cf1c15192bf73d0cc6a 100644
--- a/base/mac/scoped_nsobject.h
+++ b/base/mac/scoped_nsobject.h
@@ -15,7 +15,9 @@
 #include "base/compiler_specific.h"
 #include "base/mac/scoped_typeref.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
 @class NSAutoreleasePool;
+#endif
 
 namespace base {
 
@@ -41,11 +43,24 @@ namespace base {
 
 namespace internal {
 
+id ScopedNSProtocolTraitsRetain(__unsafe_unretained id obj)
+    __attribute((ns_returns_not_retained));
+id ScopedNSProtocolTraitsAutoRelease(__unsafe_unretained id obj)
+    __attribute((ns_returns_not_retained));
+void ScopedNSProtocolTraitsRelease(__unsafe_unretained id obj);
+
 template <typename NST>
 struct ScopedNSProtocolTraits {
-  static NST InvalidValue() { return nil; }
-  static NST Retain(NST nst) { return [nst retain]; }
-  static void Release(NST nst) { [nst release]; }
+  static NST InvalidValue() __attribute((ns_returns_not_retained)) {

[Nico, 2016/04/04 17:53:45]

is this one needed? [nil retain] is a noop, right?

+    return nil;
+  }
+  static NST Retain(__unsafe_unretained NST nst)

[Nico, 2016/04/04 17:53:45]

`Retain() returns_not_retained` needs a comment on why returns_not_retained is
correct on a method called Retain :-)

+      __attribute((ns_returns_not_retained)) {
+    return ScopedNSProtocolTraitsRetain(nst);
+  }
+  static void Release(__unsafe_unretained NST nst) {
+    ScopedNSProtocolTraitsRelease(nst);
+  }
 };
 
 }  // namespace internal
@@ -58,7 +73,9 @@ class scoped_nsprotocol
                       internal::ScopedNSProtocolTraits<NST>>::ScopedTypeRef;
 
   // Shift reference to the autorelease pool to be released later.
-  NST autorelease() { return [this->release() autorelease]; }
+  NST autorelease() __attribute((ns_returns_not_retained)) {
+    return internal::ScopedNSProtocolTraitsAutoRelease(this->release());
+  }
 };
 
 // Free functions
@@ -82,8 +99,10 @@ class scoped_nsobject : public scoped_nsprotocol<NST*> {
  public:
   using scoped_nsprotocol<NST*>::scoped_nsprotocol;
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
   static_assert(std::is_same<NST, NSAutoreleasePool>::value == false,
                 "Use ScopedNSAutoreleasePool instead");
+#endif
 };
 
 // Specialization to make scoped_nsobject<id> work.