Fix syntax/c. Closes PR13447.

collects/racket/contract/private/misc.rkt

@@ -814,7 +814,7 @@
    #:projection list/c-chaperone/other-projection))
 
 (define/subexpression-pos-prop (syntax/c ctc-in)
-  (let ([ctc (coerce-contract 'syntax/c ctc-in)])
+  (let ([ctc (coerce-flat-contract 'syntax/c ctc-in)])
     (flat-named-contract
      (build-compound-type-name 'syntax/c ctc)
      (let ([pred (flat-contract-predicate ctc)])

collects/tests/racket/contract-test.rktl

@@ -9493,6 +9493,10 @@
               'pos
               'neg))
 
+  (test/no-error '(syntax/c (list/c #f)))
+  (contract-error-test 'syntax/c-non-flat '(syntax/c (vector/c #f))
+                       (λ (x) (regexp-match #rx"expected a flat contract" (exn-message x))))
+
 
 ;
 ;