Compiler warnings on Sierra with Xcode 9

[chdiza]

I see the following compiler warning, and I don't recall seeing it before on Sierra with Xcode 8.3.3. Apologies if it isn't new.

/stuff/bld/macvim/src/MacVim/MMTextStorage.m:994:18: warning: incompatible
      pointer types assigning to 'NSTextStorage *' from
      'NSMutableAttributedString *' [-Wincompatible-pointer-types]
    backingStore = [[NSMutableAttributedString alloc] init];
                 ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /stuff/bld/macvim/src/MacVim/MMTextStorage.m:34:
In file included from /stuff/bld/macvim/src/MacVim/MMTextStorage.h:11:
In file included from /stuff/bld/macvim/src/MacVim/MacVim.h:11:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:60:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFPropertyList.h:17:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFStream.h:20:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/include/dispatch/dispatch.h:59:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/include/os/object.h:101:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/include/objc/NSObject.h:63:1: note:
      instance method 'init' is assumed to return an instance of its receiver
      type ('NSMutableAttributedString *')
- (instancetype)init
^
1 warning generated.