Forbid non-productive recursive types.

original commit: ef2acecb96900e52c08170f81e5e69eaf9737f8d

collects/tests/typed-racket/fail/non-productive-rec.rkt

@@ -0,0 +1,6 @@
+#;
+(exn-pred 1)
+
+#lang typed/racket/base
+(define-type Hole (Rec Hole (U Number Hole)))
+(ann "aaa" Hole)

collects/tests/typed-racket/fail/non-productive-rec2.rkt

@@ -0,0 +1,19 @@
+#;
+(exn-pred 1)
+
+#lang typed/racket
+
+(define-struct: node ({left : Tree} {right : Tree}) #:transparent)
+(define-type Tree (Rec Tree (U Number Tree)))
+;; Tree = Number | (node Tree Tree)
+
+(: tree-sum (Tree -> Number))
+;; sum up all numbers in the tree
+(define (tree-sum t)
+  (cond
+    [(number? t) t]
+    [else (+ (tree-sum (node-left t)) (tree-sum (node-right t)))]))
+
+(define n (node (node 12000000000000.1 .0000000000000000000000001) 2))
+
+(tree-sum n)

collects/typed-racket/private/parse-type.rkt

@@ -200,10 +200,19 @@
               [tvar (make-F var)])
          (add-disappeared-use #'kw)
          (extend-tvars (list var)
-           (let ([t (parse-type #'t)])
-             (if (memq var (fv t))
-                 (make-Mu var t)
-                 t))))]
+           (let ([t* (parse-type #'t)])
+             ;; is t in a productive position?
+             (unless (match t*
+                       [(Union: es)
+                        (define seq-tvar (Type-seq tvar))
+                        (not (memf (λ (e) (eq? (Type-seq e) seq-tvar)) es))]
+                       [_ #t]) ; it's fine
+               (tc-error/stx
+                stx
+                "Recursive types are not allowed as members of unions directly inside their definition"))
+             (if (memq var (fv t*))
+                 (make-Mu var t*)
+                 t*))))]
       [((~and kw t:U) ts ...)
        (add-disappeared-use #'kw)
        (apply Un (map parse-type (syntax->list #'(ts ...))))]