infer a type for fields sans declared type

racket/typed/roles.rkt

@@ -156,7 +156,9 @@
      (⇒ : ★/t)
      (⇒ ν-f (τ))])
 
-(define-typed-syntax (field [x:id τ-f:type e:expr] ...) ≫
+(define-typed-syntax field
+  [(_ [x:id τ-f:type e:expr] ...) ≫
+   #:cut
    #:fail-unless (stx-andmap flat-type? #'(τ-f ...)) "keep your uppity data outta my fields"
    [⊢ e ≫ e- (⇐ : τ-f)] ...
    #:fail-unless (stx-andmap pure? #'(e- ...)) "field initializers not allowed to have effects"
@@ -166,7 +168,12 @@
    ----------------------------------------------------------------------
    [⊢ (erased (field/intermediate [x x- τ e-] ...))
       (⇒ : ★/t)
-     (⇒ ν-ep (MF))])
+      (⇒ ν-ep (MF))]]
+  [(_ flds ... [x:id e:expr] more-flds ...) ≫
+   #:cut
+   [⊢ e ≫ e- (⇒ : τ)]
+   --------------------
+   [≻ (field flds ... [x τ e-] more-flds ...)]])
 
 (define-typed-syntax (assert e:expr) ≫
   [⊢ e ≫ e- (⇒ : τ)]