(define-type … (Instance X)) breaks when X is a parametric type synonym, since Racket 8.7

[capfredf]

What version of Racket are you using?

8.8

What program did you run?

@scolobb reports on Discourse:

(define my-class%
  (class object%
    #:forall (A)
    (super-new)
    (init-field [x : A])
    (define/public (get-x) x)))

I can define a synonym for the type of this class and then use it like so:

(define-type (MyClass% A) (Class (field (x A)) (get-x (-> A))))
(: a (Instance (MyClass% Boolean)))
(define a (new (inst my-class% Boolean) [x #t]))

(define-type (MyClassInstance A) (Instance (Class (field (x A)) (get-x (-> A)))))

but the following type alias causes an error

(define-type (MyClassInstance A) (Instance (MyClass% A)))

What should have happened?

The type system shouldn't have complained.

If you got an error message, please include it here.

; Type Checker: parse error in type;
; expected a class type for argument to Instance
; given: (MyClass% A)
; in: (Instance (MyClass% A))