[Issue 19345] New: std.concurrency does not work with structs of const value type

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19345

          Issue ID: 19345
           Summary: std.concurrency does not work with structs of const
                    value type
           Product: D
           Version: D2
          Hardware: x86
                OS: Mac OS X
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

The following code:
```
@system unittest
{
    static struct Aggregate { const int a; const int[5] b; }
    static void t1(Tid mainTid)
    {
        const sendMe = Aggregate(42, [1, 2, 3, 4, 5]);
        mainTid.send(sendMe);
        mainTid.send(0, sendMe);
    }

    auto tid = spawn(&t1, thisTid);
    tid.send(1);
    auto result = receiveOnly!Aggregate();
    immutable expected = Aggregate(42, [1, 2, 3, 4, 5]);
    assert(result == expected);
}
```

Will fail to compile and yield the following error message:
```
std/concurrency.d(777): Error: cannot modify struct instance
ret.__expand_field_0 of type Aggregate because it contains const or immutable
members
```

This is because `std.concurrency` creates a struct, then assign its fields
here:
https://github.com/dlang/phobos/blob/9fcf1f1b77a3a10f5369466cce58e8af7dc8ebcf/std/concurrency.d#L773-L777

In some cases, it's possible to use `receive` as a workaround, which offers a
delegate interface.

--