Initializing an Immutable Field with Magic: The "Fake Placement New" Technique

[FeepingCreature]

On Friday, 26 July 2019 at 14:19:11 UTC, ag0aep6g wrote:
> The pain points:
> 1) The spec doesn't say clearly when union fields are 
> considered initialized.
> 2) DMD allows @safe access of (uninitialized) immutable union 
> fields.
> 3) __ctor can be called on an existing instance in @safe code. 
> That's clearly a bug.

I forgot to mention: none of this is @safe, of course. Manual 
lifetime management is almost inherently unsafe. Which is why 
Nullable is peppered with @trusted...