Weird rule for immutable

[Adam D. Ruppe]

On Saturday, 11 April 2020 at 01:45:20 UTC, Victor Porton wrote:
> But is there any practical reason that the specification 
> disallows the following?

Any code that actually uses immutable data is allowed to make 
various assumptions about it, like keeping the reference for 
later, aggressively caching, etc.

Your code doesn't break that so it works (that's actually allowed 
by "undefined behavior"; the language doesn't define what it 
does, but you, the programmer, can know things the spec doesn't), 
but in general code can do other things with it that would break 
here.

> int[] x = [];
> {
>   immutable y = cast(immutable)x;
> }
> x = [1, 2];

technically btw this code is not strictly wrong because it is the 
REFERENCE where immutability matters, and here you are replacing 
it with new data, not actually modifying the old. Just like 
`string a = "foo"; a = "bar";` is allowed despite string being 
immutable contents. But I'm pretending it means what you think it 
means.