Values are not assignable to deferred conditional types even when they are assignable to both branches.

[s0]

TypeScript Version: 4.1.0-dev.20201009 & 4.0.3

Search Terms: deferred conditional assignment not assignable to type

Code

const doSomething = <T extends boolean>(t: T) => {
  const a: string = 'foo';
  const b: string | undefined = 'bar';
  const c: T extends true ? string : (string | undefined) = 'baz' as string;
  return c;
}

Expected behavior:

(baz as string) is assignable to c as string is assignable to both branches of the deferred conditional type.

Actual behavior:

The following error is emitted:

Type 'string' is not assignable to type 'T extends true ? string : string | undefined'.ts(2322)

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-dev.20201009#code/FAYw9gdgzgLgBAEzAZTAWwKYwBYEsIDmcAvHADwAqcGAHjBhAlHAEZhgA2GAhhAHwAKGAC44FAJQk+cAN7A4ccNHjdRsAE74ipAOQAzdjoDc8xZFis1MTYTgAfOAFdGGPfgwIScHS27rjpkoWIKJUtPSMzNaOGHAA-HAaWnCiAkm2Ds4Iru4Ikrq+AF4BCupYjuoQiiYAvkA

Related Issues: #39364 #39577

[jcalz]

Duplicate of #26933 ?

[s0]

Duplicate of #26933 ?

Yep, was unable to find it because i had explicitly searched for deferred.

Closing.