fix binding table when shadowing imports

pkgs/racket-test-core/tests/racket/stx.rktl

@@ -2239,6 +2239,25 @@
                              [current-load-relative-directory (build-path dir "inner")])
                 (read i)))))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check that shadowing doesn't create an ill-formed internal
+;; representation of binding:
+
+(let ()
+  ;; Make introducers before namespace, so they have older scopes, which
+  ;; means that bindings will be attached to the namespace's scope:
+  (define i1 (make-syntax-introducer))
+  (define i2 (make-syntax-introducer))
+  (define ns (make-base-namespace))
+  (eval `(define car 0) ns)
+  (eval `(define ,(i1 (datum->syntax #f 'car)) 1) ns)
+  (eval `(define ,(i2 (datum->syntax #f 'car)) 2) ns)
+  (eval `(require racket/base) ns) ; replaces plain `car` mapping
+  (write (compile-syntax
+          #`(quote-syntax #,(parameterize ([current-namespace ns])
+                              (namespace-syntax-introduce (datum->syntax #f 'car)))))
+         (open-output-bytes)))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/syntax.c

@@ -2655,7 +2655,7 @@ static Scheme_Object *replace_matching_scopes(Scheme_Object *l, Scheme_Scope_Set
 
   p = SCHEME_CDR(p);
   while (c--) {
-    p = scheme_make_pair(SCHEME_CAR(l), p);
+    p = scheme_make_mutable_pair(SCHEME_CAR(l), p);
     l = SCHEME_CDR(l);
   }