The only useful definition is that valid GENERIC is what the
gimplifier can turn into valid GIMPLE, which is much more well-defined
;) Modulo bugs in the gimplifier of course ...But that's the whole problem! If you have a tree that the gimplifier can't correctly process, how do you determine whether it's not valid GENERIC or whether the gimplifier has a bug? Using a tautology as the definition isn't helpful in that process.
