When an Objective-C property has a (copy) attribute, the default setter
for this property performs a -copy on the object assigned.
Calling -copy on a mutable NS object such as NSMutableString etc.
produces an immutable object, NSString in our example.
Hence the getter becomes type-incorrect.
rdar://problem/21022397
Differential Revision: https://reviews.llvm.org/D27535
llvm-svn: 289554
Artem Dergachev