type-level-group-by.ts

import type { Equal, Expect } from '@type-challenges/utils'

type Assign<A, B> = {
    [K in keyof A | keyof B]: K extends keyof B
        ? B[K]
        : K extends keyof A
        ? A[K]
        : never;
}

type As<a, b> = a extends b ? a : never

type GroupBy<xs, key, acc = {}> = 
  xs extends [infer first, ...infer rest]
    ? key extends keyof first
      ? GroupBy<
          rest,
          key,
          Assign<acc, {
            [K in As<first[key], PropertyKey>]: [
                ...(first[key] extends keyof acc ? As<acc[first[key]], any[]> : []),
                first
            ]
          }>
        >
      : GroupBy<rest, key, acc>
    : acc
  

type res1 = GroupBy<[[1,2,3], [1,3,4], [2,3,4]], 0>

type res2 = GroupBy<[
  { role: 'admin', name: 'A' }, 
  { role: 'admin', name: 'B' }, 
  { role: 'user', name: 'C' }, 
  { role: 'user', name: 'D' }, 
  { role: 'user', name: 'E' }
], 'role'>