Environment information

CLI:
  Version:                      2.3.9
  Color support:                true

Platform:
  CPU Architecture:             aarch64
  OS:                           macos

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  TERM:                         xterm-256color
  JS_RUNTIME_VERSION:           v25.2.1
  JS_RUNTIME_NAME:              node
  NODE_PACKAGE_MANAGER:         bun/1.3.5

Biome Configuration:
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Workspace:
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What happened?

Consider the following code:

async function* plus(iterable: AsyncIterable<number>, n: number) {
  for await (const v of iterable) {
    yield v + n;
  }
}

function* plusThreeEverythingAndThenPlusTwoEverything(
  iterable: AsyncIterable<number>,
) {
  yield* plus(iterable, 3);
  yield* plus(iterable, 2);
}

https://biomejs.dev/playground/?code=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%2BACwACgApACAAewAKACAAIAB5AGkAZQBsAGQAKgAgAHAAbAB1AHMAKABpAHQAZQByAGEAYgBsAGUALAAgADMAKQA7AAoAIAAgAHkAaQBlAGwAZAAqACAAcABsAHUAcwAoAGkAdABlAHIAYQBiAGwAZQAsACAAMgApADsACgB9AAoA

EDIT: the code above is a bit silly because the second plus(…) call doesn't do anything — the iterator is already consumed. See below for an example that can't be trivially rewritten.

Biome errors and tells me that the async keyword is unnecessary:

Remove this async modifier, or add an await expression in the function. Async functions without await expressions may not need to be declared async.

However, removing the async would cause the code to break, because we are yielding to an async iterable.

Expected result

No error.

I'm not sure exactly what the best course of action here is. The simplest would be to count yield* similar to an await call in an async function — that's probably what I would do to eliminate false positives. But yield* … can also be used to yield to sync generators (even from inside async functions), so this would allow false negatives.¹

However, the code is concise and idiomatic for combinators of iterables. It doesn't feel right to rewrite it or to add a permanent comment for Biome to ignore it.

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