TypeScriptは`interface Foo { ... }`と`type Foo = { ... }`を区別するっぽい！？

types.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 }

type Expected = { x: number | null }

// OK
const _proofNullable: Equal<Nullable<{ x: number }>, Expected> = true

type Foo = {
  x: number
}

// OK
const _proofNullable1: Equal<Nullable<Foo>, Expected> = true

interface FooI {
  x: number
}

// 2344: Type 'FooI' does not satisfy the constraint 'Record<string, unknown>'.  Index signature is missing in type 'FooI'.
const _proofNullable0: Equal<Nullable<FooI>, Expected> = true