expander: fix taint propagation

A taint on a syntax object without scopes to propagate was propagated
incorrectly to a nested syntax object that also has pending scopes to
propagate.

racket/src/expander/syntax/scope.rkt

@@ -383,9 +383,9 @@
                                                                             (syntax-scopes sub-s)
                                                                             (syntax-shifted-multi-scopes sub-s)
                                                                             (syntax-mpi-shifts sub-s))])
-                                   (struct-copy syntax sub-s
-                                                [scope-propagations+tamper (tamper-tainted-for-content
-                                                                            (syntax-content sub-s))]))))])
+                                   (struct-copy/t syntax sub-s
+                                                  [tamper (tamper-tainted-for-content
+                                                           (syntax-content sub-s))]))))])
         (set-syntax-content! s new-content)
         (set-syntax-scope-propagations+tamper! s (tamper-propagated (if (propagation? prop)
                                                                         (propagation-tamper prop)

racket/src/expander/syntax/taint.rkt

@@ -10,7 +10,9 @@
          syntax-arm
          syntax-disarm
          syntax-rearm
-         syntax-taint)
+         syntax-taint
+
+         struct-copy/t)
 
 (define-syntax struct-copy/t
   (syntax-rules (syntax tamper)