Wrong indexOf type in array

[aczekajski]

TypeScript Version: 3.4.0 and 3.5.0-dev.20190418

Search Terms: indexOf, array union, ts(2345)

Code
There seems to be an error in type of indexOf function in Array interface. It seems to be too strict, not allowing to search for values that might or might not be inside the array.

type AB = 'A' | 'B';
type CD = 'C' | 'D';
type ABCD = AB | CD;

const fn1 = (t: AB[], v: ABCD) => t.indexOf(v); // v might be "B" so it is a perfectly valid indexOf usage

const fn2 = (t: AB[], v: CD) => t.indexOf(v); // AB and CD have nothing in common, this might be an error

const fn3 = (t: ABCD[], v: AB) => t.indexOf(v); // this is ok

Expected behavior:
indexOf inside fn1 shouldn't throw an error because there is a non-empty intersection between array values type and searched value type

Actual behavior:
indexOf inside fn1 throws the following error:

Argument of type 'ABCD' is not assignable to parameter of type 'AB'.
  Type '"C"' is not assignable to type 'AB'.ts(2345)

Playground Link: TypeScript Playground

[RyanCavanaugh]

Effective duplicate of #26255

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.