title: pinescript 'simple' form parameter has different performance between built-in function and user-defined function
body: <p>code like this:</p>
  <pre><code>indicator(&quot;simple&quot;)
  var x = 1
  fun(simple int len)=&gt;len
  y = fun(x)
  y1 = ta.ema(close, x)
  y2 = ta.ema(close, 1)
  x := bar_index+1
  plot(y)
  plot(y1-y2)
  </code></pre>
  <p><strong>fun</strong> is user-defined function and has a <strong>simple form</strong> parameter named <strong>'len'</strong>,
  and built-in function <strong>ta.ema</strong>'s second parameter is also a <strong>simple form</strong>,
  but in my code above, they have different performance:
  <strong>ta.ema</strong>'s simple parameter feeze the value of simple parameter on first bar, value is 1, so in my code y1 == y2, it perform like official doc described;
  but <strong>fun</strong>'s simple parameter not! it doesn't feeze the value, it perform like a series, so the y value is not always 1, it equal bar_index and changed on every bar!</p>
  <p>simple' form parameter has different performance between built-in function and user-defined function, and how to use simple form on user-defined function? is it a system bug?</p>