remove recursive type checks from Kysely and subtypes by koskimas · Pull Request #1963 · kysely-org/kysely

koskimas · 2026-07-31T09:20:27Z

Remove Kysely assignment type check recursion. This one comes with a rather big caveat, that we might not want:

let k1: Kysely<{ a: A, b: B }>
let k2: Kysely<{ a: A }>
k2 = k1 // This no longer works!

Opus says it only worked so far because typescript gave up with the infinite recursion. If it was able to follow it through, the result would've been an error:

On the assignability change you approved: Kysely<{a,b}> ↔ Kysely<{a}> no longer compiles in either direction. Worth knowing why it used to: the compiler didn't trust the variance it measured and fell back to a structural comparison, where methods match bivariantly. It was never a variance the library could have declared — I confirmed TypeScript rejects out DB with TS2636 because selectFrom takes keyof DB. Users who relied on it need $pickTables/$omitTables.

So do we want to keep that feature, which only works because typescript gives up over infinite recursion (assuming Opus is right)?

This is on top of #1962

vercel · 2026-07-31T09:20:33Z

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
kysely	Skipped		Aug 2, 2026 6:06am

pkg-pr-new · 2026-07-31T09:21:24Z

kysely_koa_example
```
npm i https://pkg.pr.new/kysely@1963
```

commit: 594062b

koskimas · 2026-07-31T09:21:44Z

+// intersection, the second of these cost 1.3M instantiations on TypeScript 7.
+bench('ReadonlyKysely assignable to ReadonlyKysely', () => {
+  return acceptsReadonlyKysely(readonlyKysely)
+}).types([26, 'instantiations'])


This used to cost 1.3M instantations 😬

github-actions · 2026-07-31T09:22:52Z

⏱️ TypeScript Benchmark Results

✅ ⏱️  No benchmark changes detected.

koskimas · 2026-07-31T09:45:42Z

+// wider builder stand in for a narrower one, which is bivariance, and bivariance
+// can't be spelled as an annotation. See `test/typings/test-d/generic.test-d.ts`
+// for what keeps those in check instead.
+export class Kysely<in out DB>


Rather excessive AI babbling... Do we want this here at all?

Probably not.

Its better if this stuff is kept in PR descriptions - agents love getting context from gh api - when necessary.

igalklebanov · 2026-08-02T05:52:35Z

 bench('Kysely assignable to Kysely', () => {
  return acceptsKysely(plainKysely)
-}).types([11784, 'instantiations'])
+}).types([2, 'instantiations'])


remove recursive type checks from Kysely and subtypes

dabc8e9

koskimas requested a review from igalklebanov July 31, 2026 09:20

koskimas commented Jul 31, 2026

View reviewed changes

koskimas marked this pull request as ready for review July 31, 2026 12:36