The issue is not that the plugin wouldn't be gpl; it would. The concern is that the AST dumped is not code, but an artifact, and thus can be used by proprietary software and make proprietary software better.
I could write a program that compiles your gcc-produced AST for a new architecture, and not GPL it because the AST itself probably isn't GPL protected. That's the fear.
I could write a program that compiles your gcc-produced AST for a new architecture, and not GPL it because the AST itself probably isn't GPL protected. That's the fear.