error on correct code using `Identity a = a`

[lue-bird]

When a function annotation includes a type that contains

type alias Identity a =
    a

with an inferred type variable

map : (a -> b) -> Identity a -> Identity b
map change =
    change

elm falsely throws an error

Something is off with the body of the `map` definition:

    map change = change
                 ^^^^^^
This `change` value is a:

    a -> b

But the type annotation on `map` says it should be:

    Identity a -> Identity b

(on "why and when would you could want Identity")

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