Weird rule for immutable

[Steven Schveighoffer]

On 4/10/20 9:45 PM, Victor Porton wrote:
> On Saturday, 11 April 2020 at 01:26:28 UTC, Adam D. Ruppe wrote:
>> On Saturday, 11 April 2020 at 01:19:29 UTC, Victor Porton wrote:
>>> Why this is disallowed?
>>
>> Immutable means it NEVER changes. If you want it to just temporarily 
>> not change, that's what const is for.
> 
> Yes, specification reads this way by me, too.
> 
> But is there any practical reason that the specification disallows the 
> following?
> 
> int[] x = [];
> {
>    immutable y = cast(immutable)x;
> }
> x = [1, 2];

So let me modify your example to address Adam's point:

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

x[] = 5;

And let's do something else:

void foo(const(int)[] z) {writeln(z);}

foo(x);

Theoretically, the compiler could assume that x's data has become 
immutable and by logical deduction, write "[1, 2]".

It's usually the case that if you invoke undefined behavior, the 
compiler can do anything it wants under the given assumptions.

-Steve