TR: Produce better error messages with values. Closes PR11772.

original commit: 8a2c9b4682786686e38cc7df431efd38e803383f

collects/tests/typed-scheme/fail/pr11772.rkt

@@ -0,0 +1,20 @@
+#lang typed/racket
+
+(require racket/list)
+
+(define-type expression 'expression)
+
+
+(: foo (expression -> expression))
+(define (foo expr)
+  (define-values (a b)
+    (for/fold: : (values (Listof Symbol) expression)
+               ((remaining-funs : (Listof Symbol) empty)
+                (body : expression 'expression))
+               ((fun : Symbol empty))
+               (if (empty? (filter even? '(1 2))) ;non-trival list
+                   (values remaining-funs body)
+                   (values (cons fun remaining-funs body)))))
+
+  ;error is on line above. paren should be moved before body
+  (error 'dont-care))

collects/typed-scheme/typecheck/check-below.rkt

@@ -93,6 +93,9 @@
          (tc-error/expr "Expected result with filter ~a and ~a, got ~a" f (print-object o) t1)
          (tc-error/expr "Expected ~a, but got ~a" t2 t1))
      t1]
+    [((? Type? t1) (tc-results: ts2 fs os))
+       (tc-error/expr "Expected one value, but got ~a" (length ts2))
+       t1]
     [((? Type? t1) (? Type? t2))
      (unless (subtype t1 t2)
        (tc-error/expr "Expected ~a, but got ~a" t2 t1))

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

@@ -528,7 +528,16 @@
     ;; handle apply specially
     [(#%plain-app apply f . args) (tc/apply #'f #'args)]
     ;; special case for `values' with single argument - we just ignore the values, except that it forces arg to return one value
-    [(#%plain-app values arg) (single-value #'arg expected)]
+    [(#%plain-app values arg)
+     (match expected
+      [#f (single-value #'arg)]
+      [(tc-result1: tp)
+       (single-value #'arg expected)]
+      [(tc-results: ts)
+       (single-value #'arg) ;Type check the argument, to find other errors
+       (tc-error/expr #:return expected
+         "wrong number of values: expected ~a but got one"
+          (length ts))])]
     ;; handle `values' specially
     [(#%plain-app values . args)
      (match expected

collects/typed-scheme/typecheck/tc-expr-unit.rkt

@@ -452,7 +452,10 @@
   (define t (if expected (tc-expr/check form expected) (tc-expr form)))
   (match t
     [(tc-result1: _ _ _) t]
-    [_ (tc-error/stx form "expected single value, got multiple (or zero) values")]))
+    [_ (tc-error/expr
+          #:stx form
+          #:return (or expected (ret (Un)))
+     "expected single value, got multiple (or zero) values")]))
 
 ;; type-check a list of exprs, producing the type of the last one.
 ;; if the list is empty, the type is Void.