This is Rule 1 of the “Initializer Chaining” rules as specified in the Swift Programming Guide, which reads:
Rule 1: Designated initializers must call a designated initializer from their
immediate superclass.
https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html
Emphasis mine. Designated initializers cannot call convenience initializers.
There is a diagram that goes along with the rules to demonstrate what initializer “directions” are allowed:
