[paavohtl]

It gives a type error, because strings are not assignable to string literals.

https://www.typescriptlang.org/play?#code/FAEwpgxgNghgTmABAM...

However, if you check or assert that the returned value is one of the accepted literals, compiler accepts it:

https://www.typescriptlang.org/play?#code/FAEwpgxgNghgTmABAM...

This is one of the classic examples of flow-sensitive typing in TS.