Update 2016-06-28: The iOS icon templates provided by Apple as part of the iOS Human Interface Guidelines section of its developer website have been updated to the correct shape.
Last week Apple posted templates for iOS icons on its developer website. Designer Michael Flarup noticed something was a little off about the templates. It turns out that the templates aren't the 'squircle' icon shape that is created when developers submit an app to the App Store. Instead, the templates are rounded rectangles. The difference is small, but as Flarup explains with a neat GIF that highlights the difference between the two shapes:
using the wrong canvas can have consequences for your design. Apple will crop your icon in the squircle shape, so you’d better not try to do anything precise in the templates they provide.
Fortunately, Flarup and others have created their own squircle templates that are linked in his article.