Try harder to find types for loop variables.

- use `find-annotation' more
 - recognize (let ([x y]) ...)

original commit: bdbb6d48e62e8009154a095f1fc5752766ab34fe

collects/tests/typed-scheme/succeed/for-in-range.rkt

@@ -0,0 +1,3 @@
+#lang typed/racket
+
+(for: ([i : Integer (in-range 10 0 -1)]) i)

collects/typed-scheme/typecheck/find-annotation.rkt

@@ -45,13 +45,19 @@
 
 ;; expr id -> type or #f
 ;; if there is a binding in stx of the form:
-;; (let ([x (reverse name)]) e)
+;; (let ([x (reverse name)]) e) or
+;; (let ([x name]) e)
 ;; where x has a type annotation, return that annotation, otherwise #f
 (define (find-annotation stx name)
   (define (find s) (find-annotation s name))
   (define (match? b)
     (syntax-parse b
       #:literals (#%plain-app reverse)
+      [c:lv-clause
+       #:with n:id #'c.e
+       #:with (v) #'(c.v ...)
+       #:fail-unless (free-identifier=? name #'n) #f
+       (or (type-annotation #'v) (lookup-type/lexical #'v #:fail (lambda _ #f)))]
       [c:lv-clause
        #:with (#%plain-app reverse n:id) #'c.e
        #:with (v) #'(c.v ...) 

collects/typed-scheme/typecheck/tc-app.rkt

@@ -235,12 +235,14 @@
        (tc/rec-lambda/check form args body lp (cons acc-ty ts) expected)
        expected)]
     ;; special case when argument needs inference
-    [_     
+    [(_ (body*) _)
      (let ([ts (for/list ([ac (syntax->list actuals)]
                           [f (syntax->list args)])
-                 (let ([infer-t (type-annotation f #:infer #t)])
+                 (let* ([infer-t (or (type-annotation f #:infer #t)
+                                     (find-annotation #'body* f))])
                    (if infer-t
-                       (check-below (tc-expr/t ac) infer-t)
+                       (begin (check-below (tc-expr/t ac) infer-t)
+                              infer-t)
                        (generalize (tc-expr/t ac)))))])
        (tc/rec-lambda/check form args body lp ts expected)
        expected)]))