full_code.ts

class Base {
  base = 'base'
}

class A extends Base {
  a = 'a'
}
class B extends Base {
  b = 'b'
}

class C {
  c = 'c'
}


const fn = <
  T extends typeof Base,
  Instance extends InstanceType<T>
>(Cls: T & (new () => Instance), prop: keyof Instance,
) => new Cls()[prop];

const result = fn(A, 'base') // ok

const result2 = fn(B, 'b') // ok

const result3 = fn(Base, 'base') // ok

const result4 = fn(A, 'b') // expected error

const result5 = fn(B, 'a') // expected error

const result6 = fn(C, 'x') // expected error