Manifest constant class instances

[lngns]

On Sunday, 4 November 2018 at 21:08:56 UTC, kinke wrote:
> On Sunday, 4 November 2018 at 20:57:26 UTC, lngns wrote:
>> On Sunday, 4 November 2018 at 20:50:45 UTC, kinke wrote:
>>> On Sunday, 4 November 2018 at 19:28:14 UTC, lngns wrote:
>>>> On Sunday, 4 November 2018 at 19:20:53 UTC, kinke wrote:
>>>>> On Sunday, 4 November 2018 at 19:02:33 UTC, lngns wrote:
>>>>>> I can understand for pointers to struct, as the pointer 
>>>>>> will be invalid at runtime, but, unless I am mistaken, 
>>>>>> classes are not concerned by pointer semantics.
>>>>>
>>>>> A class reference is a pointer too, so using it at runtime 
>>>>> would be invalid too.
>>>>
>>>> Yes but given it works with static constants I would assume 
>>>> the compiler already abstracts away this point. Otherwise 
>>>> there would be a mismatch between what the compiler 
>>>> allocates and what the runtime allocates.
>>>> Am I wrong?
>>>
>>> The instance for a static class reference lives in the data 
>>> segment of the binary and is not GC-allocated.
>>
>> Yes this was actually my point.
>> Both the compiler and the program refer to the same object, so 
>> the pointer is necessarily valid at runtime.
>
> `enum c = new C()` doesn't imply that the instance lives at 
> runtime too. `static const c = new C()` on the other hand does. 
> This should arguably work though, but doesn't:
>
> ```
> class C {}
> static immutable c = new C();
> enum p = c;
>
> void main()
> {
>     import std.stdio;
>     writeln(p);
> }
> ```

I think one could argue that `static const c = new C();` does not 
make the object visible to the compiler and the CTFE engine, but 
merely the pointer. Yet, it does.
It looks to me the case we are discussing arises from the class 
reference semantics.
Adding that manifest constants are immutable, I think whether 
`enum c = new C();` implies the instance lives at runtime or not 
is an optimization and not a language feature. (unless there are 
non-aliasing guarantees for const objects?)

That is why I think such code should be valid.