Conditional Types

[sinclairzx81]

This PR implements conditional type mapping for TypeBox. It achieves this by replicating TypeScript's structural equivalence rules, and enables for the additional utility types Extract<...> and Exclude<...>. This functionality was considered for 0.24.0 release however required additional updates to inline with the new TypeGuard namespace.

// ----------------------------------------------------
// TypeScript
// ----------------------------------------------------

type A = {
  x: number,
  y: number,
  z: number
}

type B = {
  x: number,
  y: number
}

type X = A extends B ? true : false // true

type Y = B extends A ? true : false // false

// ----------------------------------------------------
// TypeBox
// ----------------------------------------------------

import { Conditional } from '@sinclair/typebox/conditional'
import { Type } from '@sinclair/typebox'

const A = Type.Object({
  x: Type.Number(),
  y: Type.Number(),
  z: Type.Number()
})

const B = Type.Object({
  x: Type.Number(),
  y: Type.Number()
})

const X = Conditional.Extends(A, B,  // { const: true, type: 'boolean' }
  Type.Literal(true), 
  Type.Literal(false)
)
const Y = Conditional.Extends(B, A,  // { const: false, type: 'boolean' }
  Type.Literal(true), 
  Type.Literal(false)
)

Additional

• Added ReturnType and Parameters for Function utility types.
• Added ConstructorParameters and InstanceType for Constructor utility types.