[demurgos]

It is intentional to avoid non-free projects from building on top of gcc components.

I am not familiar enough with gcc to know how it impacts out-of-tree free projects or internal development.

The decision was taken a long time ago, it may be worth revisiting it.

[janc_]

Over the years several frontends for languages that used to be out-of-tree for years have been integrated. So both working in-tree & outside are definitely possible.