is (T == Specifier)

import std.stdio;

interface Clock {
    void tellTime();
}

class AlarmClock : Clock {
    override void tellTime() {
        writeln("10:00"); 
    }
}

void myFunction(T)(T parameter) {
    static if (is (T == Clock)) {
        // If we are here then T can be used as a Clock
        writeln("This is a Clock; we can tell the time"); 
        parameter.tellTime();
        
    } else {
        writeln("This is not a Clock"); 
    }
}
void main() {
    auto var = new AlarmClock; 
    myFunction(var); 
    myFunction(42);
}