I'm pretty sure the "comptime" keyword only forces you to provide an argument constant at compile time for that particular parameter. It doesn't trigger the compile time evaluation.

That's how the constant is provided - through compile time evaluation.

Yes, but compile-time evaluation in Zig doesn't require the "comptime" keyword. Only specific cases such as compile-time type computation do (but these specific cases are not provided by compile-time function evaluation in D anyway, so language choice wouldn't make a difference here).