Detect when a ~literal is unbound

[michaelballantyne]

Like syntax-parse does here:

#lang racket

(require (for-syntax syntax/parse))

(define-syntax (m stx)
  (syntax-parse stx
    #:literals (els)
    [(_ else)
     #''TODO]))

(m els)
;; => syntax-parse: literal is unbound in phase 0 (phase 0 relative to the enclosing module) in: els

Unfortunately syntax-parse doesn't do this for ~literal patterns which is what we are using, so we may not be able to just re-use the behavior from syntax-parse. But we can go look at how Ryan implements it.