> I feel pretty confident that 20 years ago, there would have been some proprietary compilers derived from GCC if they could emit the AST in an unrestricted sor

> I feel pretty confident that 20 years ago, there would have been some proprietary compilers derived from GCC if they could emit the AST in an unrestricted sort of way. Your confidence is 100% justified, because Open64 exists. It always has been possible to emit AST from GCC by simply modifying GCC yourself to do so. GCC is a free software after all. Open64 did that, and AMD used to sell a compiler using GCC frontend together with highly optimizing proprietary backend. More on http://tirania.org/blog/archive/2004/Apr-18.html http://tirania.org/blog/archive/2004/Apr-18.html What has not been possible is not to emit AST from GCC, but to emit AST from GCC and continue to benefit from GCC changes after that.