Disallow use of attrset symbols as Struct members

Fixes [Bug #11326]

struct.c

@@ -551,6 +551,9 @@ rb_struct_s_def(int argc, VALUE *argv, VALUE klass)
     tbl = RHASH_TBL(rest);
     for (i=0; i<argc; i++) {
 	VALUE mem = rb_to_symbol(argv[i]);
+        if (rb_is_attrset_sym(mem)) {
+            rb_raise(rb_eArgError, "invalid struct member: %"PRIsVALUE, mem);
+        }
 	if (st_insert(tbl, mem, Qtrue)) {
 	    rb_raise(rb_eArgError, "duplicate member: %"PRIsVALUE, mem);
 	}

test/ruby/test_struct.rb

@@ -60,6 +60,10 @@ module TestStruct
     assert_equal(1, o.a)
   end
 
+  def test_attrset_id
+    assert_raise(ArgumentError) { Struct.new(:x=) }
+  end
+
   def test_members
     klass = @Struct.new(:a)
     o = klass.new(1)