class X has forward references

[bauss]

What could cause that error?

I cannot find anything in the documentation nor does the error 
message itself give much information.

I can't really give a good example, but I can tell as much as I 
have a few inheritances of classes using templates.

I just don't think that would be the issue.

When I try to make a minimized example it doesn't happen, so I'm 
going to assume it's either a function or a member or something 
in one of the inheritances that does it.

It would be really helpful if I knew what I was looking for, 
because it's thousands of lines of code I have to dig through 
with trial and error.