9155 - ValidDate

[darkyzhou]

Other solutions mainly use union types of string literals, I find it feasible to solve the problem without them:

type ValidDate<T extends string> = T extends `${infer M1}${infer M2}${infer D1}${infer D2}${infer Rest}` ? Rest extends '' ?
  `${M1}${M2}` extends keyof MonthDays ?
    `${D1}${D2}` extends '00' ? false :
      InRange<MonthDays[`${M1}${M2}`], `${D1}${D2}`>
  : false
: false : false;

type MonthDays = {
  '01': '31',
  '02': '28',
  '03': '31',
  '04': '30',
  '05': '31',
  '06': '30',
  '07': '31',
  '08': '31',
  '09': '30',
  '10': '31',
  '11': '30',
  '12': '31',
}

type GreaterMap = {
  '0': [],
  '1': ['0'],
  '2': ['1', '0'],
  '3': ['2', '1', '0'],
  '4': ['3', '2', '1', '0'],
  '5': ['4', '3', '2', '1', '0'],
  '6': ['5', '4', '3', '2', '1', '0'],
  '7': ['6', '5', '4', '3', '2', '1', '0'],
  '8': ['7', '6', '5', '4', '3', '2', '1', '0'],
  '9': ['8', '7', '6', '5', '4', '3', '2', '1', '0'],
}

type Greater<A extends string, B extends string> = A extends keyof GreaterMap ? Contains<B, GreaterMap[A]> : never;
type Contains<B extends string, ARR extends any[]> = ARR extends [infer Head, ...infer Rest] ?
  Eq<B, Head> extends true ? true : Contains<B, Rest>
: false;
type GreaterOrEq<A extends string, B extends string> = Greater<A, B> extends true ? true : Eq<A, B>;
type Eq<A extends any, B extends any> = A extends B ? B extends A ? true : false : false;

type InRange<R extends string, T extends string> = R extends `${infer R1}${infer R2}` ?
  T extends `${infer T1}${infer T2}` ?
    Greater<R1, T1> extends true ? true : Eq<R1, T1> extends true ? GreaterOrEq<R2, T2> : false
  : never
: never;