Test.ts

import deprecated from 'deprecated-decorator'

class Untyped {
  public set(_: string, __: any): void { }
}

class Typed<T extends object> extends Untyped {
  public assign(x: AFieldOf<T>, __: Typed[x]) { }  // 依存型させてぇ！

  @deprecated('assign')
  public set(_: string, __: any): void { }
}

type TupleToUnion<T> = T extends (infer I)[] ? I : never
type AFieldOf<T> = TupleToUnion<[keyof T]>

const x = new Typed<{ x: number }>()
x.assign('x', 10)
x.set('x', 20)
console.log(x)