"'await' has no effect on the type of this expression" from inferred void and Promise<void> union

[joshkel]

🔎 Search Terms

"'await' has no effect on the type of this expression" , "promise<void>"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Why are functions returning non-void assignable to function returning void?

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.1-rc#code/MYewdgzgLgBGIgA4wLwwBQEpUD4YG8BfAbgChSBDCATzGBgDMBXOqAS3EYXQYBsKA5gC4YAIwS8AphTDZ8pGDFCRYkgB6TgTKJNSN+AmAH4YVWvSy4CMAPQAqGABMQMdYkmQ2AN10B3EABOANYwdjYwhDAi8EhkihS+FGyqGlo6WGSEpEA

💻 Code

const noop = () => {};

async function foo(flag: boolean) {
  const execute = flag ? async () => { /* do expensive work */ } : noop;
  await execute();
}

🙁 Actual behavior

The type of execute is inferred as () => void, so TypeScript displays a spurious warning on the await execute(); line, "'await' has no effect on the type of this expression.(80007)".

🙂 Expected behavior

TypeScript infers the type as () => void | () => Promise<void>, and calling await execute() generates no warnings.

Based on the FAQ, I would expect explicitly assigning () => Promise<void> to () => void to work, but I would not expect () => void | () => Promise<void> to be inferred as () => void and correct code then generating compiler diagnostics.

Additional information about the issue

I found some possibly related issues: #44201, #43921, #49755

If this issue is actually a duplicate of one of those, I apologize.

[fatcerberus]

I believe this is unfortunate fallout of subtype reduction.

[RyanCavanaugh]

Yeah, there's not terribly much we can do here. The best solution is to annotate noop:

const noop = (): undefined => {};