Recursive type not unifying with itself

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1

0

The following code fails to compile with error:

type must be Tuple(Thing::Ish, Slice(UInt8)), not Tuple(Array(Array(Thing::Ish) | UInt8) | UInt8, Slice(UInt8))

These two types seem equivalent to me… and adding an .as(Ish) in the right place works… what am I missing? Why do these types not unify?

module Thing
    alias Ish = UInt8 | Array(Ish)

    def self.decode(bytes : Bytes) : {Ish, Bytes}
        case bytes[0]
            when 0x00..0x17
                {bytes[0], bytes + 1}
            when 0x18
                MultiItemDecoder.new(0x80, ->(x: Bytes) { Thing.decode(x) }).decode(bytes)
            else
                raise "unknown"
        end
    end

    class MultiItemDecoder(T)
        def initialize(@base : UInt8, @item_decoder : Bytes -> {T, Bytes})
        end

        def decode(bytes): {Array(T), Bytes}
            decode_some(bytes + 1, bytes[0])
        end

        def decode_some(bytes, n)
            items = n.times.map do
                item, bytes = @item_decoder.call(bytes)
                item
            end
            {items.to_a, bytes}
        end
    end
end
2

There has already been a partial answer on SO. Cross-posting is okay, but please mention it so others can connect: https://stackoverflow.com/questions/56013690/recursive-type-not-unifying-with-itself