More info in internal errors.

Fix substitution in nested ... case.

collects/typed-scheme/private/tc-utils.ss

@@ -107,7 +107,9 @@
 
 ;; raise an internal error - typechecker bug!
 (define (int-err msg . args)  
-  (raise (make-exn:fail:tc (string-append "Internal Typechecker Error: " (apply format msg args))
+  (raise (make-exn:fail:tc (string-append "Internal Typechecker Error: "
+                                          (apply format msg args)
+                                          (format "\nwhile typechecking\n~a" (syntax->datum (current-orig-stx))))
                            (current-continuation-marks))))
 
 (define-syntax (nyi stx)

collects/typed-scheme/private/type-utils.ss

@@ -40,7 +40,8 @@
                  [#:arr dom rng rest drest thn-eff els-eff
                         (begin
                           (when (and (pair? drest)
-                                     (eq? name (cdr drest)))
+                                     (eq? name (cdr drest))
+                                     (just-Dotted? name))
                             (int-err "substitute used on ... variable ~a in type ~a" name target))
                           (make-arr (map sb dom)
                                     (sb rng)