# Availability of \`Failure\` type in \`AsyncSequence\`

**URL:** <https://forums.swift.org/t/availability-of-failure-type-in-asyncsequence/75019>\
**Category:** Using Swift\
**Tags:** concurrency, asyncsequence, typed-throws\
**Created:** [September 29, 2024, 6:43pm UTC](https://forums.swift.org/t/availability-of-failure-type-in-asyncsequence/75019 "2024-09-29T18:43:10Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![aetherealtech](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/aetherealtech/32/36879_2.png) [@aetherealtech](https://forums.swift.org/u/aetherealtech)\
**Post date:** [September 29, 2024, 6:43pm UTC](https://forums.swift.org/t/availability-of-failure-type-in-asyncsequence/75019/1 "2024-09-29T18:43:10Z")

</div>

I've been playing around with `AsyncSequence` which has now been updated to have a `Failure` associatedtype, with availability restricted to the latest platform versions. But I encountered something curious. The `flatMap` operator looks like this:

```swift
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension AsyncSequence {
    ...

    @preconcurrency @inlinable public func flatMap<SegmentOfResult>(_ transform: @escaping @Sendable (Self.Element) async -> SegmentOfResult) -> AsyncFlatMapSequence<Self, SegmentOfResult> where SegmentOfResult : AsyncSequence, Self.Failure == SegmentOfResult.Failure
}

```

Notice there is no restriction to latest platform versions (it's only restricted to versions that support concurrency), but it does make use of `Failure`, which if I follow to its definition, _is_ restricted to the latest platform versions.

I discovered this after I upgraded Xcode (I can't remember if the problem surfaced before or after changing the language version to 6), in a library where I wrote a `flatten` operator (i.e. `flatMap { $0 }`). The compiler complained that there were no matching overloads, and that the correct one isn't a candidate because it requires the constraint on `Failure`. So I added that constraint, and got an error that `Failure` isn't available on the target platform version. I eventually had to change the return type to `AsyncThrowingFlatMapSequence`, and write another version of `flatten` that is restricted to latest platform versions.

It was possible previously for it to return `AsyncFlatMapSequence` because `AsyncSequence` was `@rethrows`, but now this seems to not be possible anymore, which means it's a breaking change.

How is it possible that this extension is written so that it is available on older platforms but uses `Failure`? Is that a magic capability available only to the standard or "core" modules like `_Concurrency`?

Xcode let slip while showing possible overloads that the function is actually marked with `@_alwaysEmitIntoClient`, so I thought maybe that is why it works (that it would defer the restriction up to whoever imports the module), but I tried adding that and it didn't get rid of the error.