Understand signature of opOpAssign in std/complex.d

[Q. Schroll]

On Saturday, 9 November 2019 at 13:26:12 UTC, Adam D. Ruppe wrote:
> On Saturday, 9 November 2019 at 12:30:46 UTC, René Heldmaier 
> wrote:
>> The part i don't understand is "is(C R == Complex!R)".
>> What does that mean?
>
> That's checking the if the template argument C is a case of 
> Complex!R, while at the same time declaring a symbol R to hold 
> the inner type.

What's the difference of
     is(C R == Complex!R)
to
     is(C == Complex!R, R)
then?