I actually found the bug by mistake. I was going to override a method in a subclass and got the error. The strange thing is that I had copied the method from the super class and it didn't work anyway. Then I saw, in the super class, that I didn't have a return type on the method. I had just missed declaring a return type, the intention was not to use type inference. I guess I use Ruby too much :)
https://github.com/D-Programming-Language/dmd/blob/master/src/parse.c#L2598
