TS 7: Property 'not' does not exist on type Assertion<void, T>

Start at the exported Assertion type declaration and the Vitest assertion typings described in the issue. Reproduce the failure with the three files shown and run pnpm exec tsc --noEmit. The work is done when expect("a").not.toBe("b") typechecks under TypeScript 7.0.2 without affecting the existing runtime API.

Describe the bug

On TypeScript 7.0.2, expect(value).not.toBe(...) fails to typecheck:

error TS2339: Property 'not' does not exist on type 'Assertion<void, string>'.

Runtime is fine. Vitest 5.0.0 documents .not and the Jest/Chai APIs both use it.

Assertion does not declare not. It is supposed to come from the circular mapped type over Chai.Assertion:

type VitestAssertion<A, R extends void | Promise<void>, T = unknown> = {
  [K in keyof A]: A[K] extends Chai.Assertion
    ? Assertion<R, T>
    : /* ... */
} & ((type: string, message?: string) => Assertion<R, T>)

export interface Assertion<R extends void | Promise<void> = void, T = unknown>
  extends VitestAssertion<Chai.Assertion, R, T>,
    JestAssertion<R, T>,
    ChaiMockAssertion<R, T>,
    Matchers<R, T> { /* ... */ }

Under TypeScript 7 that mapping drops .not. Declaring not: Assertion<R, T> on Assertion restores it.

Happy to send a one-line PR if you want that explicit field.

Reproduction

Three files. Then pnpm exec tsc --noEmit.

package.json:

{
  "type": "module",
  "devDependencies": {
    "typescript": "7.0.2",
    "vitest": "5.0.0"
  }
}

tsconfig.json:

{
  "compilerOptions": {
    "strict": true,
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "skipLibCheck": true
  },
  "include": ["repro.ts"]
}

repro.ts:

import { expect } from "vitest"

expect("a").not.toBe("b")

Expected: tsc --noEmit succeeds (same as TypeScript 5 with Vitest 4).

Actual:

repro.ts:3:13 - error TS2339: Property 'not' does not exist on type 'Assertion<void, string>'.

System Info

System:
  OS: macOS (Darwin arm64)
Binaries:
  Node: 24.14.1
  pnpm: 12.3.1
npmPackages:
  typescript: 7.0.2
  vitest: 5.0.0
  vite: 8.2.2

Used Package Manager

pnpm

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