ICE with `pub type` with direct use of default type parameter

[libfud]

Changing the line

type Bigrat<T= BigRational> = T;

to

pub type BigRat<T = BigRational> = T;

causes the compiler to fail with the following backtrace:

error: internal compiler error: found unmapped ID in worklist: 4791
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/buil
d/src/libsyntax/diagnostic.rs:162                                                           

stack backtrace:
   1:     0x7f976c495900 - rt::backtrace::imp::write::he132d79389e358c8AGp::v0.11.0.pre
   2:     0x7f976c49d030 - failure::on_fail::h5cc622b2b6de7c92N1p::v0.11.0.pre
   3:     0x7f976c8b7410 - unwind::begin_unwind_inner::hfdc3321d430c4be5QRd::v0.11.0.pre
   4:     0x7f976b34a430 - unwind::begin_unwind::h15411178358729660202::v0.11.0.pre
   5:     0x7f976b34ae70 - diagnostic::Handler::bug::h3a98fff19f29d074Y3b::v0.11.0.pre
   6:     0x7f976cf0c9d0 - driver::session::Session::bug::h5fe77e17d8bc65ccrlw::v0.11.0.pre
   7:     0x7f976d6bd3a0 - middle::reachable::find_reachable::h816b66fbd6806c79nO4::v0.11.0.p
re                                                                                          
   8:     0x7f976d83e270 - driver::driver::phase_3_run_analysis_passes::hcb3b633763f249271mv:
:v0.11.0.pre                                                                                
   9:     0x7f976d8367e0 - driver::driver::compile_input::h86d461aeb26e888dgbv::v0.11.0.pre
  10:     0x7f976d901290 - driver::run_compiler::h3debb6db2f3b527cOSx::v0.11.0.pre
  11:     0x7f976d9011a0 - driver::main_args::closure.98525
  12:     0x7f976d91cd00 - driver::monitor::closure.99615
  13:     0x7f976d917c40 - task::TaskBuilder::try::closure.99378
  14:     0x7f976f9702a0 - task::spawn_opts::closure.7148
  15:     0x7f976c8b3fc0 - task::Task::run::closure.5303
  16:     0x7f976c91ea20 - rust_try
  17:     0x7f976c8b6a00 - unwind::try::heae236e404445881fGd::v0.11.0.pre
  18:     0x7f976c8b3e40 - task::Task::run::h1536f4cce70d9777VVc::v0.11.0.pre
  19:     0x7f976f970040 - task::spawn_opts::closure.7121
  20:     0x7f976c8b5fd0 - thread::thread_start::h2c9b863e7dad2f1ekdd::v0.11.0.pre
  21:       0x3832407e70 - start_thread
  22:       0x3831cf4db9 - clone
  23:                0x0 - <unknown>

[huonw]

Minimal example:

#![feature(default_type_params)]

pub type X<T = int> = T;

fn main() {}

It doesn't ICE with pub type X<T = int> = Option<T>;.