Help in reviewing druntime PR

[Paul Backus]

On Thursday, 9 December 2021 at 09:47:10 UTC, Stanislav Blinov 
wrote:
> ...I mean, it looks like this would have to be ported into 
> Phobos as well, seeing as there's still this weird code 
> duplication for those symbols going on. Though I would rather 
> see those things removed from Phobos and aliased into core, 
> deprecated, then deleted for good.
>
> But I could add the Phobos change too if there's more eyes 
> there.

Something like this is already in Phobos, under the name 
[`std.traits.hasNested`][1].

However, the behavior is not exactly the same. The Phobos version 
recurses into classes, even though they are reference types, 
leading to results like the following:

```d
void example()
{
     import std.traits;

     class Nested {}
     static struct NotNested
     {
         Nested nested;
     }

     pragma(msg, hasNested!NotNested); // true
}
```

I'm not really sure what we should do about this. The Phobos 
behavior seems obviously incorrect to me, but "fixing" it is a 
potentially-breaking change. Maybe the right thing to do is to 
add the fixed version to Phobos as 
`std.traits.hasContextPointers` and mark `hasNested` as 
outdated/unrecommended in the documentation. Thoughts?

[1]: https://dlang.org/phobos/std_traits.html#hasNested