[Frognar]

What

TP-11, card 6.2 of milestone M6 (1.0.0-beta.1), in the shape decided in #16.

• Validated.CombineAsync(t1, …, tN, combine) for N = 2…8 Task<Validated<…>> inputs: the asynchronous Combine. The tasks run concurrently, and every error of every input comes back in argument order (V7). It is generated from one template, like Combine.
• Every input is a task. Two spikes showed that a mix of bare validations and tasks cannot infer types (C# does not infer through conversions); the builder and ZipAsync alternatives were dropped (see the comment in feat: add CombineAsync (TP-11) #16). A synchronous check goes in as Task.FromResult(...), or the synchronous checks are combined on their own and joined with Zip.
• R8 semantics: missing tasks or a missing combiner throw at once, by parameter name. When every task has already succeeded, the result is complete at once and nothing is allocated. Otherwise Task.WhenAll observes every task: a failed task comes out as itself, a canceled one cancels the result, an uninitialized result gives ArgumentException, and a null from the combiner gives InvalidOperationException. The library never resumes on the caller's synchronization context.

Tests: CombineAsyncProperties (every arity ≡ awaiting each input + Combine, with completed and yielding tasks; a completed result for completed tasks; a failure at any position of every arity comes out as itself; cancellation; null and uninitialized results; no posts to a counting SynchronizationContext for any arity; missing tasks by name for every position). The first Stryker run left 83 survivors. 34 were ConfigureAwait on already completed tasks, gone now that the results are read in a non-async helper. The rest showed missing per-arity tests, which are now in place. Allocation lines: CombineAsync(2) and (8) with completed tasks, 0 B. CombineAsyncContracts: tasks infer; a bare validation hits CS0411 and Task.FromResult fixes it; the split + Zip pattern compiles. Stryker locally: 100 %. README "Async" with the split pattern; AOT smoke extended (a yielding task, accumulation, joining with Zip).

Closes #16

Checklist

• Tests cover the change (properties first, examples only where a property does not fit)
• dotnet test --solution dotValid.slnx is green locally
• Public API changes have XML docs

🤖 Generated with Claude Code

https://claude.ai/code/session_012weRUz3QcTAaQw32ZmQkoK