need for flag for incompatible-changes

[Joe Buck]

> > Clearly, if the names of the functions change (due to namespaces) but
> > their calling conventions did not (same argument types etc.) then the
> > linker can map old names to new names if it is told to do so.
> 
> Isn't this the kind of de(re)mangling that <collect> was invented for?

collect does not remangle symbols.  It finds symbols that match a
pattern (file-scope ctor/dtor calls).  It pipes errors from the linker
through the demangler, and that's about it.

If someone wants to "sign up" for a task that benefits only those who
distribute binary-only libraries or executables, fine, but otherwise
I doubt if any of the developers will want to bother.