I don't understand why this code doesn't compile:

http://dpaste.dzfl.pl/dfd8df7f80ad

On Wednesday, 15 October 2014 at 19:29:27 UTC, Jack Applegame wrote:
> I don't understand why this code doesn't compile:
>
> http://dpaste.dzfl.pl/dfd8df7f80ad

that should be immutable(ubyte)[] not immutable ubyte[] which is equivalent to immutable(ubyte[]). You can't overwrite immutable data, hence the error.

On Wednesday 15 October 2014 21:29, Jack Applegame wrote:

> I don't understand why this code doesn't compile:
> 
> http://dpaste.dzfl.pl/dfd8df7f80ad

Immutability issues with unions ring a bell for me. This may be related to issue 12885 [1] which is the result of dmd pull #2665 [2] which even mentions std.date.SysTime.

Here's a reduced version of your test case:
---
struct Rebindable
{
    union
    {
        int m;
        immutable int i;
    }
    void opAssign(Rebindable another) {}
}

struct Foo3
{
    // Remove the union or _timezone and dmd accepts it.
    union
    {
        int m;
        immutable int i;
    }
    Rebindable _timezone;
}

void main()
{
    Foo3 foo3;
    foo3 = Foo3();
}
---

[1] https://issues.dlang.org/show_bug.cgi?id=12885
[2] https://github.com/D-Programming-Language/dmd/pull/2665

On Wednesday 15 October 2014 22:13, John Colvin wrote:

> On Wednesday, 15 October 2014 at 19:29:27 UTC, Jack Applegame wrote:
>> I don't understand why this code doesn't compile:
>>
>> http://dpaste.dzfl.pl/dfd8df7f80ad
> 
> that should be immutable(ubyte)[] not immutable ubyte[] which is
> equivalent to immutable(ubyte[]). You can't overwrite immutable
> data, hence the error.

Then

  Foo1 foo1; foo1 = Foo1(); // compiles

shouldn't compile either.

On Wednesday, 15 October 2014 at 21:28:05 UTC, Nils Boßung wrote:
> On Wednesday 15 October 2014 22:13, John Colvin wrote:
>
>> On Wednesday, 15 October 2014 at 19:29:27 UTC, Jack Applegame
>> wrote:
>>> I don't understand why this code doesn't compile:
>>>
>>> http://dpaste.dzfl.pl/dfd8df7f80ad
>> 
>> that should be immutable(ubyte)[] not immutable ubyte[] which is
>> equivalent to immutable(ubyte[]). You can't overwrite immutable
>> data, hence the error.
>
> Then
>
>   Foo1 foo1; foo1 = Foo1(); // compiles
>
> shouldn't compile either.

Good point. That's an accepts-invalid IMO.

Nonetheless:
union { string a; immutable ubyte[] b; }
is broken, it is effectively casting away immutability.
union { string a; immutable(ubyte)[] b; } is the correct approach.