"Input array signature parameter SV_Position cannot be indexed dynamically."

Heyho! I’m writing a DX11 shader and my code cannot compile saying “Input array signature parameter SV_Position cannot be indexed dynamically.” I have no idea what can cause this. Anybody can help?
It’s a little bit large code though that’s why I’m not uploading it in the first place. (there’s many uncommented garbage left)
on second thought:

https://dl.dropboxusercontent.com/u/1218206/DeferredBaseSimple.fx

the problematic part is in pixel shader where i’m getting the barycentric coordinate of the pixel on the current triangle and calculating the curvature based on the triangle normals. if i comment that part out everything is alright. not the best and tidiest code so sorry for crying out in agony :D