fix Struct's setter arity

https://github.com/ruby/ruby/pull/5131/files#diff-b2553d23e6b1fe76e20608d06c25f6acca06279100f1a9c24febcd79a82fac3cR2689

proc.c

@@ -2684,7 +2684,7 @@ method_def_min_max_arity(const rb_method_definition_t *def, int *max)
             return 0;
           case OPTIMIZED_METHOD_TYPE_STRUCT_ASET:
             *max = 1;
-            return 0;
+            return 1;
 	  default:
 	    break;
 	}

test/ruby/test_struct.rb

@@ -497,6 +497,20 @@ module TestStruct
     assert_equal(42, x.public_send("a"))
   end
 
+  def test_arity
+    klass = @Struct.new(:a)
+    assert_equal 0, klass.instance_method(:a).arity
+    assert_equal 1, klass.instance_method(:a=).arity
+
+    klass.module_eval do
+      define_method(:b=, &klass.new.method(:a=).to_proc)
+      alias c= a=
+    end
+
+    assert_equal 1, klass.instance_method(:b=).arity
+    assert_equal 1, klass.instance_method(:c=).arity
+  end
+
   def test_parameters
     klass = @Struct.new(:a)
     assert_equal [], klass.instance_method(:a).parameters