TypeAssign.ts

// recursive magic
export type Tail<T extends any[]> = T['length'] extends 0
  ? never
  : ((...b: T) => void) extends (a: any, ...b: infer I) => void
    ? I
    : [];

// Type of { ...L, ...R }
// Properties in L that don't exist in R plus R
type _TypeAssignWorker<L, R> = Pick<L, Exclude<keyof L, keyof R>> & R;
type _TypeAssign<
  Dest,
  SourceTuple extends unknown[],
  > = SourceTuple[0] extends void
  ? Dest
  : _TypeAssign<_TypeAssignWorker<Dest, SourceTuple[0]>, Tail<SourceTuple>>;

// Type of Object.assign(Dest, ...SourceTuple)
export type TypeAssign<
  DestOrMergeList,
  S2 = void,
  S3 = void,
  S4 = void,
  S5 = void,
  S6 = void,
  S7 = void,
  S8 = void,
  S9 = void,
  S10 = void,
  S11 = void,
  S12 = void,
  S13 = void,
  S14 = void,
  S15 = void,
  S16 = void,
  > = DestOrMergeList extends unknown[]
  ? _TypeAssign<Record<string, never>, DestOrMergeList>
  : _TypeAssign<
    DestOrMergeList,
    [S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14, S15, S16]
    >;