Weird rule for immutable

[Walter Bright]

On 4/10/2020 6:45 PM, Victor Porton wrote:
> But is there any practical reason that the specification disallows the following?
> 
> int[] x = [];
> {
>    immutable y = cast(immutable)x;
> }
> x = [1, 2];

Because Data Flow Analysis would needed to prove there are not other mutations 
to x within the lifetime of y. DFA is slow and complex, and doesn't seem worth 
it for this case.