Construct immutable member in derived class

[Timoses]

On Wednesday, 4 April 2018 at 18:11:12 UTC, Jonathan M Davis 
wrote:
> On Wednesday, April 04, 2018 16:05:52 Timoses via
>>
>> ```
>> class A
>> {
>>      immutable int i;
>>
>>      this(){}
>> }
>>
>> class B : A
>> {
>>      this()
>>      {
>>           this.i = 3;
>>          super();              // <- specifically calling
>>                                //    super constructor 
>> afterwards
>>      }
>> }
>>
>> void main()
>> {
>>   auto b = new B;
>> }
>> ```
>
> That code doesn't compile - at least not with dmd master. It 
> gives these two errors:
>
> q.d(5): Error: constructor `q.A.this` missing initializer for 
> immutable
> field i
> q.d(12): Error: cannot modify immutable expression this.i
>
> So, it's an error that the base class doesn't initialize the 
> immutable member, and it's an error for the derived class to 
> try to assign to it.
>
> - Jonathan M Davis

I know, should have mentioned it. The question is why, however?
A rule like the above would force all derived classes to 
initialize the immutable variable from the base class (if they 
were allowed to).
Why aren't they?