Improve error message for calls to 'sort' when inference fails.

original commit: b124d68f402ce99ddec58482be40ce279f3f4a04

collects/tests/typed-scheme/fail/sort.rkt

@@ -0,0 +1,5 @@
+#;
+(exn-pred exn:fail:syntax? #rx".*Cannot infer.*Please add more type annotations.*")
+#lang typed/racket
+(define ss '("one" "two" "three")) ; (Listof String)
+(sort ss string<?)

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

@@ -124,6 +124,9 @@
     ;; error type is a perfectly good fcn type
     [((tc-result1: (Error:)) _) (ret (make-Error))]
     ;; otherwise fail
+    [((tc-result1: (and f-ty (Poly: ns (Function: arrs)))) _)
+     (tc-error/expr #:return (ret (Un))
+                    "Cannot infer type instantiation for type ~a. Please add more type annotations" f-ty)]
     [((tc-result1: f-ty) _)
      (tc-error/expr #:return (ret (Un))
                     "Cannot apply expression of type ~a, since it is not a function type" f-ty)]))