Align Async.Await with fslang-suggestions #840 (raise TaskCanceledException on cancel)

[T-Gro]

Aligns Async.Await with fslang-suggestions #840, as requested in dotnet/fsharp#19785 (comment) and tracked by #676.

What

On cancellation, both Await overloads (Task<'T> and Task) now call the exception continuation with a TaskCanceledException (ec) instead of the cancellation continuation (cc):

-elif task.IsCanceled then cc (TaskCanceledException ())+elif task.IsCanceled then ec (TaskCanceledException ())

Why

With cc, a cancelled Task propagated as an async cancellation, so a try ... with :? TaskCanceledException -> placed around the Await would not catch it. With ec it is surfaced as an ordinary exception, catchable locally — matching C# await and every AwaitTaskCorrect-derived implementation. The doc comment already described this behaviour.

Note: Async.map2/map3/zip build on Await, so a purely-cancelled input now surfaces as a TaskCanceledException rather than a cancellation. Where cancellation is combined with a fault, the fault still wins (cancellation dropped in Task.map* before reaching Await).

Tests

• Asyncs.fs zip tests build their cancellation input directly (they previously relied on Await of a cancelled Task yielding a cancellation).
• Added Task.fs coverage asserting a cancelled Task/Task<'T> await raises a TaskCanceledException catchable by try/with.

Based on tgro-fsharp10, so the diff against master also includes that branch's net10/net11 build-tracking work.

[Copilot]

Pull request overview

Aligns Async.Await’s cancellation semantics with fslang-suggestions #840 by surfacing a canceled Task as a catchable TaskCanceledException via the exception continuation (rather than treating it as async cancellation), and updates tests accordingly.

Changes:

• Updated Async.Await (for Task<'T> and Task) to route IsCanceled through the exception continuation (ec) with TaskCanceledException.
• Updated async zip tests to use a truly canceled Async (via cancellation continuation) rather than relying on awaiting a canceled Task.
• Added tests asserting a canceled Task/Task<'T> awaited via Async.Await raises a TaskCanceledException catchable by try/with.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/FSharpPlus/Extensions/Extensions.fs	Changes Async.Await cancellation path to raise TaskCanceledException via exception continuation.
tests/FSharpPlus.Tests/Asyncs.fs	Adjusts zip test inputs to produce genuine async cancellation independent of Async.Await behavior.
tests/FSharpPlus.Tests/Task.fs	Adds coverage to ensure awaiting canceled tasks raises TaskCanceledException catchable by try/with.

Suppressed comments (1)

src/FSharpPlus/Extensions/Extensions.fs:200

• Same as the generic overload: prefer TaskCanceledException(task) over TaskCanceledException() so the thrown exception is associated with the awaited task (matching typical await semantics) and preserves the Task reference for diagnostics.

 if task.IsFaulted then
let e = Unchecked.nonNull task.Exception
if e.InnerExceptions.Count = 1 then ec e.InnerExceptions[0]
else ec e
elif task.IsCanceled then ec (TaskCanceledException ())
else sc ())

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[wallymathieu]

Thanks @T-Gro for the contribution! 😃