Expression for: foo is derived from Clock

[Nick Treleaven]

On Wednesday, 18 February 2026 at 14:49:56 UTC, Brother Bill 
wrote:
> ```Object foo``` is a class instance, perhaps null, perhaps a 
> Clock or AlarmClock or Vegetable.
>
> What is the expression to ask the boolean question:
>   Is foo of type Clock or a type derived from Clock, such as 
> AlarmClock?

Use a cast:
https://dlang.org/spec/expression.html#cast_class

> I tried the obvious ```foo is Clock```, but this did not 
> compile.
> ```
> Error: type `Object` is not an expression
>        bool isClock = (o is Clock)
>                             ^
> ```

That use of `is` is a different expression:
https://dlang.org/spec/expression.html#identity_expressions