syntax/parse: change unbound literal error message

collects/syntax/private/stxparse/runtime.rkt

@@ -612,7 +612,9 @@ An Expectation is one of
 ;; FIXME: change to normal 'error', if src gets stripped away
 (define (check-literal id phase ctx)
   (unless (identifier-binding id phase)
-    (raise-syntax-error #f "unbound identifier not allowed as literal" ctx id)))
+    (raise-syntax-error #f
+                        (format "literal is unbound in phase ~s" phase)
+                        ctx id)))
 
 ;; free-identifier=?/phases : id phase-level id phase-level -> boolean
 ;; Determines whether x has the same binding at phase-level phase-x

collects/syntax/private/stxparse/sc.rkt

@@ -163,7 +163,7 @@
                (begin-for-syntax/once
                 (for ([x (syntax->list #'(external ...))])
                   (unless (identifier-binding x 0)
-                    (raise-syntax-error #f "unbound identifier not allowed as literal"
+                    (raise-syntax-error #f "literal is unbound in phase 0"
                                         (quote-syntax #,stx) x))))))))]))
 
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