improve hash/c error message when non-flat-key applied to non-equal?-hash

Author: bennn

typed-racket-lib/typed-racket/utils/hash-contract.rkt

@@ -23,6 +23,9 @@
   (and/c hash? hash-weak? (hash/c/check-key k v #:immutable #f)))
 
 (define (hash/c/check-key k v #:immutable [immutable 'dont-care])
-  ;; TODO if (flat-contract? k), then make a contract that produces a "good"
-  ;;      error message given a hashtable that is not a `hash-equal?`
-  (hash/c k v #:immutable immutable))
+  (if (flat-contract? k)
+    (hash/c k v #:immutable immutable)
+    (and/c (flat-named-contract
+            "hash-equal? (because the key contract is not a flat contract)"
+            hash-equal?)
+           (hash/c k v #:immutable immutable))))

typed-racket-test/succeed/issue-625.rkt

@@ -11,7 +11,7 @@
 
 (require 't rackunit)
 
-(define err-regexp #rx"expected equal\\?-based hash")
+(define err-regexp #rx"hash-equal\\?.*key contract is not a flat contract")
 
 (check-exn err-regexp
   (λ () (give-me-a-hash (hasheqv))))