TS2.9 narrows MyType|undefined to undefined (regression from 2.8)

[webmaster128]

TypeScript Version:

• typescript@2.8.4 ok
• typescript@2.9.2 bug
• typescript@3.0.0-dev.20180626 bug

Search Terms:

Code

declare const AddressSymbol: unique symbol;
type AddressString = typeof AddressSymbol & string;

function printout(data: AddressString | undefined) {
  console.log(data); // type of `data` is always undefined since TS 2.9
}

printout(undefined); // ok

const a = "foo" as AddressString;
printout(a); // broken since TS 2.9

Expected behavior: the code compiles

Actual behavior: since TypeScript 2.9, the compilations fails with:

$ ./node_modules/typescript/bin/tsc example.ts
example.ts:11:10 - error TS2345: Argument of type 'AddressString' is not assignable to parameter of type 'undefined'.

11 printout(a); // broken in TS 2.9
            ~

Playground Link: http://www.typescriptlang.org/play/#src=declare%20const%20AddressSymbol%3A%20unique%20symbol%3B%0D%0Atype%20AddressString%20%3D%20typeof%20AddressSymbol%20%26%20string%3B%0D%0A%0D%0Afunction%20printout(data%3A%20AddressString%20%7C%20undefined)%20%7B%0D%0A%20%20console.log(data)%3B%0D%0A%7D%0D%0A%0D%0Aprintout(undefined)%3B%20%2F%2F%20ok%0D%0A%0D%0Aconst%20a%20%3D%20%22foo%22%20as%20AddressString%3B%0D%0Aprintout(a)%3B%20%2F%2F%20broken%20since%20TS%202.9

Related Issues: #25179

[ahejlsberg]

This is working as intended and is an effect of #23672. The type checker now more effectively removes intersection types with empty value domains (such as symbol & string) when they're included in union types. Since it isn't possible to construct values that are simultaneously symbol and string, the type symbol & string is effectively the same as never and the type checker treats it accordingly.

[webmaster128]

Thank you very much for the explanation, @ahejlsberg! Makes sense so far. We only cast to those things in order to get strict string aliases and they can indeed not be constructed.

But shouldn't a be of type never too in the following lines?

declare const AddressSymbol: unique symbol;
type AddressString = typeof AddressSymbol & string;
const a = "foo" as AddressString;