Generic helper with constrained table set rejects string column refs (ReferenceExpression invariance?)

[briandastous]

Versions

• Kysely: 0.28.9
• TypeScript: 5.9.3
• Node: (not relevant)

Repro

1. npm i kysely typescript
2. Create tsconfig.json:

  {
    "compilerOptions": {
      "target": "ES2022",
      "module": "NodeNext",
      "moduleResolution": "NodeNext",
      "strict": true,
      "noEmit": true,
      "types": []
    }
  }

3. Create repro.ts:

  import type { SelectQueryBuilder } from "kysely";

  interface DB {
    person: { id: number; name: string };
    pet: { id: number; owner_id: number };
  }

  // Attempt at a reusable helper that requires the "person" table to be present.
  function wherePersonHasName<DBT extends DB, TB extends keyof DBT, O>(
    qb: SelectQueryBuilder<DBT, "person" | TB, O>,
  ): SelectQueryBuilder<DBT, "person" | TB, O> {
    // Expect this to be valid because "person" is explicitly included in TB.
    return qb.where("person.name", "=", "alice");
  }

  // Usage with a concrete builder that *does* include "person".
  declare const concreteQuery: SelectQueryBuilder<DB, "person" | "pet", { id: number }>;
  wherePersonHasName(concreteQuery);

4. npx tsc -p tsconfig.json

Actual

TS2345: Argument of type 'string' is not assignable to parameter of type 'ReferenceExpression<DBT, TB | "person">'.

Expected

The helper should typecheck because "person" is explicitly included in TB and "person.name" is a valid column reference.

Notes

• Inlining the same where("person.name", ...) at the call site compiles.
• The failure appears related to SelectQueryBuilder being invariant in TB / ReferenceExpression in a generic context, which makes it hard to write reusable, type-safe query helpers without assertions or sql.ref.
• This blocks DRY filters that require certain tables to be present.