// Utility types:

type GetProps = TBase extends new (props: infer P) => any ? P : never
type GetInstance = TBase extends new (...args: any[]) => infer I ? I : never
type MergeCtor = new (props: GetProps & GetProps) => GetInstance & GetInstance


// Usage:
// bypass the restriction and manually type the signature
function GeometryMixin(Base: TBase) {
  // key 1: assert Base as any to mute the TS error
  const Derived = class Geometry extends (Base as any) {
    shape: 'rectangle' | 'triangle'
    constructor(props: { shape: 'rectangle' | 'triangle' }) {
      super(props)
      this.shape = props.shape
    }
  }

  // key 2: manually cast type to be MergeCtor
  return Derived as MergeCtor
}