function assertNever(x: never): never {
  throw new Error('Should never occur.')
}

type StudentMessage =
  | 'get-grades'
  | 'get-best-grade'
  | 'add-grade'

type MessageFunction<TMessage extends StudentMessage> =
  TMessage extends 'get-grades'
  ? () => number[]
  : TMessage extends 'get-best-grade'
  ? () => number
  : TMessage extends 'add-grade'
  ? (grade: number) => void
  : never

function Student(grades: number[]){
  const _grades = [...grades];

  return function<TMessage extends StudentMessage>(message: TMessage): MessageFunction<TMessage> {
    switch(message){
      case 'get-grades':
        return () => _grades;

      case 'get-best-grade':
        return () => Math.min(..._grades);

      case 'add-grade':
        return (grade: number) => {_grades.push(grade)};

      default:
        assertNever(message)
    }
  }
}

我得到以下错误：

Type '() => number[]' is not assignable to type 'MessageFunction<TMessage>'.
Type '() => number' is not assignable to type 'MessageFunction<TMessage>'.
Type '(grade: number) => void' is not assignable to type 'MessageFunction<TMessage>'.

我不明白，为什么TypeScript确实推断出了正确的类型，但仍然在switch语句中的return语句中抛出Error。