func execute<T: Collection>(withCollection: T) where T.Index == Int, T.SubSequence: Collection { 
}

struct Foo<Base: Collection> where Base.Index == Int, Base.SubSequence: Collection {
    let base: Base
    
    subscript(idx: Int) -> Base.SubSequence {
        return base[0..<idx]
    }
}

class Bar {
    func run<C: Collection>(_ arr: C) where C.Index == Int, C.SubSequence: Collection {
        let foo = Foo(base: arr)

        print(type(of: foo[2])) // ArraySlice<UInt8>
        execute(withCollection: foo[2]) // error: generic parameter 'T' could not be inferred
        // foo[2] is a Collection because Foo.Base.SubSequence: Collection.
    }
}

let arr = [1,2,3,4,5] as [UInt8]
Bar().run(arr)