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
