How to do dynamic dispatch?

[anthonylatsis]

@CTMacUser

But why? Does the extension's version of the initializer shadow over the base version when both have the same signature? Could I have a version for Collection in the base definition, one for BidirectionalCollection in an extension that's defined for bidirectional collections, and a third for RandomAccessCollection in yet another extension that's defined for random-access collections?

The initializer in the extension has a different generic signature (don't forget that the constraints on U are different). The one whose signature most accurately satisfies the type you passed as an argument is statically resolved as the one to be called. That should answer your last question (yes).