Understand signature of opOpAssign in std/complex.d

[René Heldmaier]

Hi,
i'm currently implementing a dual number datatype. Dual numbers 
are very similar to complex numbers, but instead i (i^2 = -1) you 
have epsilon (epsilon^2 = 0).
This sounds strange but it is really useful for something called 
"automatic derivation". I will probably explain it in more detail 
when i'm ready to publish it as a dub package...

Because of the similarity to complex numbers i looked at how they 
are implemented in std/complex.d, to learn from how it is done 
there.

I came across a function signature which i don't understand:

ref Complex opOpAssign(string op, C)(const C z)
         if ((op == "+" || op == "-") && is(C R == Complex!R))

The part i don't understand is "is(C R == Complex!R)".
What does that mean?