Doug Gregor 23b8cd18e7 Retain type sugar for extension declarations that name generic typealiases. ...

When extending a type via a generic typealias, where the type parameters of
the underlying nominal type line up precisely with those of the
generic typealias and its specialization of the underlying nominal
type (a so-called "pass-through" typealias in the new code), maintain
type sugar in the extension declaration.

This new type sugar enables inference of type requirements from the
generic typealias, which is both useful by itself (it lets the type
requirements on generic typealiases be meaningful for extensions like
they are elsewhere), and also addresses a source-compatability
regression where an extension of `CountableRange` will now infer the
requirement `Bound: Comparable`.

Fixes SR-6907 / rdar://problem/29066394.

2018-03-22 23:57:30 -07:00

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