Add support for basic polymorphic classes

original commit: 91729c060cd431cb3b438aa6f0d3572ed456ab22

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/check-class-unit.rkt

@@ -161,6 +161,8 @@
   (match (and expected (resolve expected))
     [(tc-result1: (and self-class-type (Class: _ _ _ _ _)))
      (do-check form #t self-class-type)]
+    [(tc-result1: (Poly-names: ns body-type))
+     (check-class form (ret body-type))]
     [#f (do-check form #f #f)]
     [_ (check-below (do-check form #f #f) expected)]))
 

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/unit-tests/class-tests.rkt

@@ -973,5 +973,23 @@
        (class: object%
          (super-new)
          (: x Integer)
-         (init-field x))))))
+         (init-field x))))
+
+   ;; test polymorphic class
+   (check-ok
+     (: c% (All (A) (Class (init-field [x A]))))
+     (define c%
+       (class: object%
+         (super-new)
+         (init-field x)))
+     (new (inst c% Integer) [x 0]))
+
+   ;; fails due to ill-typed polymorphic class body
+   (check-err #:exn #rx"Expected A, but got Positive-Byte"
+     (: c% (All (A) (Class (init-field [x A]))))
+     (define c%
+       (class: object%
+         (super-new)
+         (init-field x)
+         (set! x 5))))))