More precisely, the introduction of SE-0142 and SE-0157 has made canonical type computation into an undecidable problem. I'll begin with the necessary background information before presenting my argument, and then outlining the next steps.  Canonical types Consider these two protocols, stripped down from the real implementations in the standard library:  protocol IteratorProtocol {   associatedtype Element }  protocol Sequence {   associatedtype Iterator : IteratorProtocol   associatedtype Eleme...