証明できてる？

Nullable.ts

/**
 * Proves that A and B is the same types.
 */
export type Equal<A, B> = A extends B ? (B extends A ? true : never) : never

/**
 * Makes all fields of A to `<its-type> | null`.
 */
export type Nullable<A extends Record<string, unknown>> = { [K in keyof A]: A[K] | null }

const _proofNullable: Equal<Nullable<{ x: string }>, { x: string | null }> = true