Doug Gregor 0652bb7abe Always infer AsyncSequence.Failure from AsyncIteratorProtocol.Failure ...

The newly-introduced associated type `AsyncSequence.Failure` must
always be equivalent to the `Failure` type of the
`AsyncIteratorProtocol`. If the `AsyncSequence` type itself defines a
nested `Failure` type (say, for another purpose), associated type inference
would pick it and reject the `AsyncSequence`, causing a source compatibility
problem.

Work around the issue in two ways. First, always infer the type
witness for `AsyncSequence.Failure` from the type witness for
`AsyncIteratorProtocol.Failure`, so they can't be out of sync. This
means that we'll never even consider a nested `Failure` type in the
`AsyncSequence`-conforming type. This hack only applies prior to Swift 6.

Second, when we have inferred a `Failure` type and there is already
something else called `Failure` within that same nominal type, don't
print the inferred typelias into a module interface because it will
cause a conflict.

Fixes rdar://123543633.

2024-02-28 13:49:50 -08:00

1.9 KiB

Raw Blame History

View Raw