Fix bug for read-syntax on ( . args)

* module/ice-9/boot-9.scm: Capture syntax?.
* module/ice-9/read.scm (read-syntax): Avoid re-annotating objects.
* test-suite/tests/reader.test ("read-syntax"): Add test.

module/ice-9/boot-9.scm

@@ -4656,8 +4656,11 @@ R7RS."
 ;;;
 
 (call-with-values (lambda ()
-                    (include-from-path "ice-9/read.scm")
-                    (values read read-syntax))
+                    ;; Capture syntax? binding, later removed from root
+                    ;; module.
+                    (let ((syntax? syntax?))
+                      (include-from-path "ice-9/read.scm")
+                      (values read read-syntax)))
   (lambda (read* read-syntax*)
     (set! read read*)
     (set! read-syntax read-syntax*)))

module/ice-9/read.scm

@@ -877,7 +877,17 @@
 (define* (read-syntax #:optional (port (current-input-port)))
   (define filename (port-filename port))
   (define (annotate line column datum)
-    (datum->syntax #f ; No lexical context.
-                   datum
-                   #:source (vector filename line (1- column))))
+    ;; Usually when reading compound expressions consisting of multiple
+    ;; syntax objects, like lists, the "leaves" of the expression are
+    ;; annotated but the "root" isn't.  Like in (A . B), A and B will be
+    ;; annotated but the pair won't.  Therefore the usually correct
+    ;; thing to do is to just annotate the result.  However in the case
+    ;; of reading ( . C), the result is the already annotated C, which
+    ;; we don't want to re-annotate.  Therefore we avoid re-annotating
+    ;; already annotated objects.
+    (if (syntax? datum)
+        datum
+        (datum->syntax #f ; No lexical context.
+                       datum
+                       #:source (vector filename line (1- column)))))
   (%read port annotate syntax->datum))