patch 8.2.3005: Vim9: using a void value does not give a proper error message

Problem:    Vim9: using a void value does not give a proper error message.
Solution:   Give a clear error message. (clodes #8387)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 6b51bc7..d7fb1df 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2967,6 +2967,20 @@
       assert_equal([1, 2, 3], sorted)
   END
   CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+    def RetVoid()
+    enddef
+    RetVoid()->byte2line()
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1031:')
+
+  lines =<< trim END
+    def RetVoid()
+    enddef
+    RetVoid()->byteidx(3)
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1031:')
 enddef