type narrowing doesn’t work when using a non-type-literal field as a tag

Inside the test function, b type does not narrow to string when a type is string.
Why? I don’t understand why it behaves this way.

Type narrowing on private members results in Property does not exist on type ‘never’ error

When I try to use type narrowing on a private member, I get a typescript error

Typescript: allow typeof check narrowing when comparing against a const

I want the following: