Type widening for T or Promise<T>

Bug Report

🔎 Search Terms

Type widening, promise, generic type widening.

🕗 Version & Regression Information

This is the behavior in every version I tried.

⏯ Playground Link

Playground link

💻 Code

type Cb3 = <Res extends string | Promise<string>>(cb: () => Res) => Res

const c: Cb3 = {} as any;

// this don't widen the type, type of t31 is "1"
const t31 = c(() => {
  return '1';
});

// but here is the problem, type of t32 is Promise<string>
const t32 = c(async () => {
  return '1';
});

🙁 Actual behavior

Type of t32 is Promise<string>

🙂 Expected behavior

Type of t32 is Promise<"1">, by analogy with type of t31

Start with the linked TypeScript Playground and compare generic callback inference in the synchronous and async examples. Trace where the Promise case loses the literal type; done means the async call is inferred as Promise<"1"> while the synchronous result remains unchanged.