ICE with pattern guards

[catamorphism]

This code causes an ICE in trans::alt::compile_submatch (where it calls option::get):

enum pat { pat_ident(option<uint>) }

fn f(pat: pat) -> bool { true }

fn num_bindings(pat: pat) -> uint {
    alt pat {
      pat_ident(_) if f(pat) { 0 }
      pat_ident(none) { 1 }
      pat_ident(some(sub)) { sub }
    }
}

fn main() {}

I've tried to fix it, but I just don't understand the code that compiles pattern matches. Comments would be helpful.

[bblum]

We ran into a similar patterning ICE during ICFP (#2928), when doing disjunctive patterns with different variable names, I think? (i.e., "foo(a) | bar(b) { .. }") efaust determined the code was incorrect and the compiler should've errored, and it won't ICE on correct code.

Hopefully related. Tarball forthcoming.

[catamorphism]

If it doesn't ICE on correct code, then you found a different bug, because the code above is correct.

[catamorphism]

This is indeed a dup of #3121.