The only known condfail scenario is inheriting from
a protocol P that inherits from a reparentable one R.
We already guard P if it mentions R in its inheritance clause.
So older compilers will simply report that "P" is missing in the
interface, which is a better error message than virally guarding
everything mentioning P. It's an ABI break anyway to do that
without introducing a `@reparented` extension, which needs a guard.
In theory, older compilers shouldn't have too much go wrong if they
were to ignore `@reparentable`, though the RequirementMachine and
witness tables will look different.
rdar://174263176
The `@reparented` was missing and a typealias
was being synthesized unexpectedly, creating
an issue when typechecking the interface later.
There's no fundamental reason why typealiases
cannot be supported to say the same thing as
the same-type requirement, but I think the
same-type requirement is always needed to be
written on the extension, one way or another.
For now I've chosen to only go with an
explicitly-written same-type requirement.
Kavon Farvardin
Becca Royal-Gordon