Please tell me this is a bug?

[Almighty Bob via Digitalmars-d]

D version 2.066.1

int a = 42;
float b = 19;

a = b; // Error: cannot implicitly convert expression (b) of type 
float to int

a = a+b; // Error: cannot implicitly convert expression (b) of 
type float to int

but...

a += b; // Compiles with no ERROR!

Please tell me that's a bug?