Hover display for non-homomorphic mapped types renders numeric-string keys indistinguishably from numeric keys

🔎 Search Terms

mapped type hover, numeric string key display, key quoting, homomorphic mapped type

🕗 Version & Regression Information

This is the behavior in every version I tried.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAglC8UDeBtA0lAlgOygRgF0AuKNAXwG4AoAehqgYD0B+K0SKAIQWXS1wDkeAcVKVa9JsyA

💻 Code

type A = {[K in 1]: K};
//   ^? type A = { 1: 1; }
type B = {[K in '1']: K};
//   ^? type B = { 1: "1"; }

🙁 Actual behavior

A and B have keys of different types (1 vs "1"), but the hover renders both keys identically as 1. In B, the display shows key 1 mapping to value "1" — the key and value originate from the same K, so they must be the same type, yet the display suggests they're not.

🙂 Expected behavior

The printer generally prefers the simpler key representation when it's unambiguous (abc rather than "abc"), but numeric literals are themselves valid keys, so numeric-looking strings need to stay quoted to avoid collapsing into a different type. This safeguard is applied correctly in the direct case:

type Direct = {'1': 'a'};
//   ^? type Direct = { '1': "a"; }

but is lost on the non-homomorphic mapped-type path. B's expected display is { '1': "1"; }.

Additional information about the issue

A related issue (#47214) discussed this for a simpler repro and was closed with the rationale that mapped-type results go through a normalization path without an original spelling to echo. That rationale fits cases where quoting is purely cosmetic; here the unquoted form renders B in a way that obscures which type its keys actually are, so the quoting isn't a style preference but part of what distinguishes string-keyed from number-keyed types in the display.

Note: {[K in keyof T]: ...} renders correctly because it matches the homomorphic mapped type pattern, which preserves the original key representation. Constraints that don't match this pattern, force the general normalization path where the quoting is lost.

Commencez par la reproduction dans Playground et comparez les affichages au survol des mapped types direct, homomorphic et non-homomorphic décrits dans l’issue. Lisez le chemin de normalisation abordé dans l’issue #47214 et vérifiez que les clés de chaîne ressemblant à des nombres restent entre guillemets ; c’est terminé lorsque B est rendu comme { '1': "1"; } sans modifier l’affichage des clés numériques.