truly surprising error message from TR

What version of Racket are you using?

Welcome to DrRacket, version 7.0.0.20--2018-09-12(-/f) [3m].

What program did you run?

This student program:

#lang typed/racket

(define (subst [ft: BTree] [sy : Symbol] [rt : BTree])
  (match ft
    [(Node l r) (Node (subst l sy rt) (subst r sy rt))]
    [(Leaf sy) rt]
    [(Leaf s) (Leaf s)]))

What should have happened?

An error message highlighting some larger area, and text that makes a bit more sense.

If you got an error message, please include it here.

The highlighted region is the [sy : Symbol], and the error message is as follows:

define: expected more terms starting with expression
parsing context:
while parsing optional lambda argument
while parsing lambda-formals
while parsing curried-formals in: (sy : Symbol)

(yes, I see what the actual error is... but honestly, I think the only plausible way to diagnose this error is to stare at the program until you see the problem.)

Reproduce the supplied typed/racket program in DrRacket and inspect the parsing context around the optional lambda argument and curried formals. Done means the reported source region and error text make the actual malformed argument declaration easier to diagnose.