Pass temporary value for Un' along in recursive call in substitute'.

Closes PR 12600.

original commit: 678941ce5aaf52a9ca62ba6270a89c892b85b516

collects/tests/typed-racket/unit-tests/subtype-tests.rkt

@@ -1,7 +1,7 @@
 #lang scheme/base
 
 (require "test-utils.rkt"
-         (types subtype convenience union)
+         (types subtype convenience union utils abbrev)
          (rep type-rep)
          (env init-envs type-env-structs)
          (r:infer infer infer-dummy)
@@ -24,6 +24,10 @@
 
 (infer-param infer)
 
+
+(define t1 (-mu T (-lst (Un (-v a) T))))
+(define t2 (unfold t1))
+
 (define (subtype-tests)
   (subtyping-tests
    ;; trivial examples
@@ -53,6 +57,8 @@
    [(-mu x (Un -Number (make-Listof x))) (-mu y (Un -Number -Symbol (make-Listof y)))]
    ;; a hard one
    [(-mu x (*Un -Number (-pair x (-pair -Symbol (-pair x (-val null)))))) -Sexp]
+   [t1 (unfold t1)]
+   [(unfold t1) t1]
    ;; simple function types
    ((Univ . -> . -Number) (-Number . -> . Univ))
    [(Univ Univ Univ . -> . -Number) (Univ Univ -Number . -> . -Number)]

collects/typed-racket/types/substitute.rkt

@@ -33,7 +33,7 @@
 ;; substitute : Type Name Type -> Type
 (define/cond-contract (substitute image name target #:Un [Un (get-union-maker)])
   ((Type/c symbol? Type?) (#:Un procedure?) . ->* . Type?)
-  (define (sb t) (substitute image name t))
+  (define (sb t) (substitute image name t #:Un Un))
   (if (hash-ref (free-vars* target) name #f)
       (type-case (#:Type sb #:Filter (sub-f sb) #:Object (sub-o sb))
                  target