Branded string literals revert to string in some cases

[mmalerba]

🔎 Search Terms

If I create a branded string literal it seems to respect the string literal in the basic case of assigning to a property, however it loses this information and just reverts to string when assigning to a property or in a template string.

It makes sense that the brand object needs to be dropped in these cases, but rather than converting to string, it should convert to the narrower literal type.

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about branded string

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAsiAyBLYEBOBDANgHgMpQgA8UA7AEwGcoLhVESBzAPigF4p8AyKAbwH0ARhnIAuXgF9xAbgBQAYwD2JGlHRsoAcnQbVVOEhQYcWjU1nylKwmJPr0UqAHpHUAO4LUAawoBCC8uAoEBttdQADABIedHEwh2coADN0RExffxUALzFosRIAVwBbATRxdR4AbXQAXTEADjKE5NTfIA

💻 Code

type MyLiteral<S extends string> = S & {_brand: {}};
const a = 'a' as MyLiteral<'a'>;

const x: 'a' = a; // works!
const y: 'a' = `${a}`; // fails!
const z: {a: number} = {[a]: 8} // fails!

🙁 Actual behavior

For y and z, the types don't match because a is converted to string rather than 'a'

🙂 Expected behavior

The types should match in all 3 cases, x, y, z

Additional information about the issue

No response