Versioned interfaces

GCC uses versioned interfaces where appropriate (e.g. libgcc, libstdc++ and libjava), but
using versioned interfaces inside the compiler itself seems likely to be both fantastically
annoying and very slow. We're not talking coupling of the level of `struct stat' here; we're
talking passes that read and modify elaborate data structures which gain and lose properties
as they flow through different passes. How on earth would you propose versioning the tree
representing a C++ function? What if someone inserts a pass in the middle which accepts an
older version and transforms it? The later passes would expect the later version, but it's not
there anymore! (This is doubly true if the representation changes in major ways.)

No, this is much too much trouble. There are some things it makes sense to version. Parse
trees and intermediate language structures is not one of them.