5.1.3 new "feature"? Branded types are preserved in `const` template literal

[conorbrandon]

Bug Report

🔎 Search Terms

branded, interpolation, 5.1.3

🕗 Version & Regression Information

This changed between versions 5.0.4 and 5.1.3

⏯ Playground Link

Playground link with relevant code

💻 Code

type S = `id_${string & {_brand: "ID"}}`;
//   ^? type S = `id_${string & { _brand: "ID"; }}`
declare const id: string & {_brand: "ID"};
const s = `id_${id}` as const;
//    ^? const s: `id_${string & { _brand: "ID"; }}`

🙁 Actual behavior

The intersected brand is preserved during string interpolation for variables and types!

🙂 Expected behavior

Pre 5.1.3, the intersected brand always got removed during string interpolation for variables and types.

This is part bug report and question. My question being in future versions, will this behavior be preserved? I wrote a clunky generic function and type to do this exact thing because I found it valuable, but I'd much rather rely on native behavior if this is going to stick around 👀

[RyanCavanaugh]

I think this was (unintentional?) fallout from #48034 / #52345

[jakebailey]

I think this was intentional? (I at least explicitly added it and tests for it, though in the course of fixing other stuff.)

Also @weswigham

[jakebailey]

Though, I'm not sure how this should interact with as const... I guess it's a noop and that's normal?

[jakebailey]

The relevant PR is #52836, note that the tests contain effectively exactly what the original post contains, a branded type inside a template.

[conorbrandon]

Ah, template literals, that's the word I was looking for (instead of string interpolation). Had a brain fart, my apologies.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[ahejlsberg]

I'm reopening this one. The original issue, #48034, was that we're not "untagging" literal types when they occur in template literal placeholders. In other words, the expectation was that

type S1 = `xyz${"a"}`;
type S2 = `xyz${"a" & { tag: any }}`;

both resolve to "xyza". Originally, S2 would resolve to string. Currently, S2 resolves to `xyz${"a" & { tag: any }`. Neither of those are right, S2 should just resolve to "xyza".