Complex numbers in lib?

[sai]

I remember the reason D has built in complex numbers is because 
it supposed to give correct results, as mentioned at the bottom 
here: https://digitalmars.com/d/1.0/cppcomplex.html

And the math checks out: https://run.dlang.io/is/b5eyuD

So I am curious why the complex class in the library was 
introduced and the builtin complex numbers are deprecated now?

And how to avoid errors like the above when using the complex 
library?

Thanks
Sai