Type demangling error in Xcode 10 beta 6 Playgrounds

Since I've been using Xcode 10 beta 6, I've gotten:

SWIFT RUNTIME BUG: unable to demangle type of field 'base'. mangled type name is '5IndexSlQz'

whenever I do experiments with collections. The actual names ("base" and "5IndexSlQz" here) differ on what I'm actually doing; as it has happened on at least two playgrounds. Has anyone else noticed this? Is there an upcoming fix yet?

This is likely:


That is a fix on master.

@Mike_Ash was looking to see if it is fixed in the 4.2 branch.