So, I did some research, and according to NSHipster , NSEnumeration is a protocol implemented by NSArray , NSSet and NSDictionary . This suggests that if you convert [assetPlaceholder] to NSArray , you can use it in the method. And, in fact, this compiles:
let enumeration: NSArray = [assetPlaceholder!] albumChangeRequest!.addAssets(enumeration)
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