default cast in opCast

[Paul Backus]

On Monday, 29 December 2025 at 20:29:16 UTC, Basile B. wrote:
> D could define that `this` cast within an `opCast` should 
> fallback to the default, i.e that ignores the operator overload.

[...]

> The problem D has right now is that everyeach possible case 
> must be handled, there's no way to fallback to the default 
> cast-exp semantics.

IMO a better solution would be to fall back to the default cast 
behavior if no overload of `opCast` matches for a given cast 
expression. So, for example:

```d
class C {}
class B : C
{
     uint member = 8;
     T opCast(T)()
     if (T == uint) // only match for cast(uint)
     {
         return member;
     }
}

void main()
{
     auto c = new C;
     assert(cast(uint) c == 8);
     B b = cast(B) c; // no match; fall back to default
}
```