Instantiation of nested structs should be allowed outside their parent scope

[Stefan Koch]

On Monday, 8 November 2021 at 01:02:11 UTC, Stanislav Blinov 
wrote:
> Just make it `@system`, and even then ONLY if something is 
> actually getting instantiated and it's not just a CTFE test. 
> Unless I'm missing something, that should be the behavior. The 
> 'cannot access frame pointer' blah SO needs to go away...
>
> Rationale: instantiating things is super useful for compile 
> time tests and inference. Nested structs complicate that to 
> wazoo with that failure.
>
> If you're not convinced, I encourage you to try and define e.g.
>
> ```d
> enum bool isNothrowCopyable(To, From) = /* ??? */
> ```
>
> without reimplementing copy initialization by hand. It should 
> be a trivial test. Just test copy-initialization of a union 
> field. But no. Not when nested structs are involved. And they 
> can get involved indirectly.

How would you want a struct to behave when it is used outside of 
it's context?
Set the context pointer to null and throw upon access?