[GSoC 2025] Templated Hooks - Weekly Update #2

[AlbertG]

This week I've worked on 
[`_d_class_cast`](https://github.com/dlang/dmd/pull/21440).

The only issue I encountered was related to the `shared` 
attribute. Since the attributes of the argument passed to the 
hook are removed (to avoid template instance cluttering), code 
from `core.atomic`, similar to this, failed to compile:
```d
void atomicStore_impl(T)(T* dest, T value)
{}

void atomicStore(T, V)(ref shared T val, auto ref shared V newval)
{
     atomicStore_impl(cast(T*)val, cast(V)newval);
}

class A {}

shared A a;
shared A b;

void main()
{
     atomicStore(a, b);
}
```

The problem with this code is that, although the expression 
`newval` passed to the hook was stripped of attributes, the 
underlying `VarExp` was not. Therefore, a simple fix was to also 
strip the attributes from the `VarExp`:
```d
if (auto v = cex.e1.isVarExp())
{
     v.var.type = v.var.type.unqualify(MODFlags.shared_);
}
```

Now some other compilation failures came up, so I defaulted to 
skipping shared access checks for `_d_cast` altogether in 
`functionParameters()`:
```d
if (!p.isReference())
{
   if (fd && fd.ident != Id._d_cast)
   {
     err |= arg.checkSharedAccess(sc);
   }
}
```

This is a cleaner solution, and is actually safe because no 
direct accesses to the casted object are performed in the hook.

The only thing left to do is to implement a suggested improvement 
that is based on the observation that `_d_class_cast` does not 
use the template argument `To` at all, so a wrapper over it, such 
as `private void* _d_class_cast(To)(Object o){ return 
_d_class_cast_impl(o, typeid(To)); }`, could be used such that no 
duplicated code is generated for each template instantiation. The 
straightforward implementation does seem to cause some weird 
issues in phobos, but I am still investigation. As soon as I 
figure it out, the PR is good to go.