Make infer unfold Mu before spliting on Unions.

typed-racket-lib/typed-racket/infer/infer-unit.rkt

@@ -521,6 +521,10 @@
           ;; constrain b1 to be below T, but don't mention the new vars
           [((Poly: v1 b1) T) (cgen (context-add context #:bounds v1) b1 T)]
 
+          ;; Mu's just get unfolded
+          [((? Mu? s) t) (cg (unfold s) t)]
+          [(s (? Mu? t)) (cg s (unfold t))]
+
           ;; constrain *each* element of es to be below T, and then combine the constraints
           [((Union: es) T)
            (define cs (for/list/fail ([e (in-list es)]) (cg e T)))
@@ -622,10 +626,6 @@
           [((Set: t) (Sequence: (list t*)))
            (cg t t*)]
 
-          ;; Mu's just get unfolded
-          ;; We unfold S first so that unions are handled in S before T
-          [((? Mu? s) t) (cg (unfold s) t)]
-          [(s (? Mu? t)) (cg s (unfold t))]
 
           ;; resolve applications
           [((App: _ _ _) _)

typed-racket-test/unit-tests/infer-tests.rkt

@@ -257,6 +257,11 @@
       #:vars '(a)
       #:result [(-seq (-v a)) (-seq (-val 'b))]]
 
+    [infer-t
+      (-lst (-val (-v a)))
+      (Un (-pair (-v a) Univ) -Null)
+      #:vars '(a)]
+
     ;; Currently Broken
     ;(infer-t (make-ListDots -Symbol 'b) (-pair -Symbol (-lst -Symbol)) #:indices '(b))
     [i2-t (-v a) N ('a N)]