Mutate immutable inside shared static constructor

[Nick Treleaven via Digitalmars-d-learn]

I've not used static constructors before, but it seems like the 
following should not be allowed:

```d
import std.stdio;
immutable int x;

@safe shared static this()
{
    x.writeln(); // 0
    x = 5;
    x.writeln(); // 5
    x = 6;
    x++;
    assert(x == 7);
}
```
Should I file a bug to require that `x` is only written to once? 
That would make it consistent with class constructors:

```d
class C
{
    immutable int x;
    this()
    {
        x = 5;
        x = 6; // error, x initialized multiple times
    }
}
```