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

Comienza con el TypeScript Playground enlazado y compara la inferencia de callbacks genéricos en los ejemplos síncrono y asíncrono. Rastrea dónde pierde el tipo literal el caso de Promise; se considera terminado cuando la llamada asíncrona se infiere como Promise<"1"> mientras que el resultado síncrono permanece sin cambios.