mul.ts

  type Range<n, range extends any[] = []> =
    range['length'] extends n ? range : Range<n, [...range, range['length']]>
  
  type Drop1<a extends any[]> =
    a extends [...infer inits, any] ? inits : []
  
  type Concat<a extends any[], b extends any[]> = [...a, ...b]
  
  type Plus<a, b> =
    [...Range<a>, ...Range<b>]['length']
  
  type MulRange<a extends any[], b extends any[]> = 
    a extends [...infer inits, any]
      ? Concat<b, MulRange<inits, b>>
      : []

  type Cast<a, b> = a extends b ? a : never
    
  type Mul<a, b> = MulRange<Range<a>, Range<b>> extends infer res
    ?  Cast<res, any[]>['length']
    : 0


  type res1 = Mul<4, 12>;