arrays - Subscript an Arrary of generics type: error: Cannot subscript a value of type '[T]' -

how solve problem?

protocol mappable {     ... } class foo {     item:anyobject } class someclass<t:mappable> {     var someobject = foo()     var items:[t]     ...     func somefunction() {         someobject.item = items[index] // error: cannot subscript value of type '[t]'     } 

i've tried adding extension subscripting [t] fail:

extension array element:mappable {     subscript(index: int) -> element? {         return indices ~= index ? self[index] : nil     } } 

update: misleading error message, please see answers below

the issue isn't subscripting. it's type conversion. swift gave misleading error message. take @ line:

someobject.item = items[index]     anyobject ^   ^ mappable 

swift doesn't implicitly convert mappable anyobject. instead must use force cast:

func somefunction() {     // assuming `index` int     someobject.item = items[index] as! anyobject }