[ICE] Compiler panics when trying to return `impl Iterator<Item = &a>`

[skeleten]

Hi,
I've run into an ICE while trying to return an iterator over references, using the conservative_impl_trait feature.
This is a short example producing the error:
https://play.rust-lang.org/?gist=35775bb4f3cc3a143e0c46a433580b94&version=nightly&backtrace=1

The backtrace it produces is

   0: rustc::session::opt_span_bug_fmt::{{closure}}
   1: rustc::session::span_bug_fmt
   2: rustc_typeck::check::FnCtxt::register_predicate
   3: <rustc::ty::fold::BottomUpFolder<'a, 'gcx, 'tcx, F> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
   4: rustc_typeck::check::check_fn
   5: rustc_typeck::check::typeck_tables
   6: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables<'tcx>>::try_get
   7: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables<'tcx>>::get
   8: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::item_tables
   9: rustc_typeck::check::typeck_item_bodies
  10: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::try_get
  11: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::get
  12: rustc_typeck::check_crate
  13: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  14: rustc::ty::context::TyCtxt::create_and_enter
  15: rustc_driver::driver::phase_3_run_analysis_passes
  16: rustc_driver::driver::compile_input
  17: rustc_driver::run_compiler
  18: std::panicking::try::do_call
  19: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  20: <F as alloc::boxed::FnBox<A>>::call_box
  21: std::sys::imp::thread::Thread::new::thread_start
             at /checkout/src/liballoc/boxed.rs:650
             at /checkout/src/libstd/sys_common/thread.rs:21
             at /checkout/src/libstd/sys/unix/thread.rs:84
  22: start_thread
  23: clone

My rustc --version --verbose is

rustc 1.17.0-nightly (fd182c401 2017-03-13)
binary: rustc
commit-hash: fd182c4010b5aee72d070b15e90c98cb0fdc3776
commit-date: 2017-03-13
host: x86_64-unknown-linux-gnu
release: 1.17.0-nightly
LLVM version: 3.9

[skeleten]

I might be a bit to eager to post.
Duplicate of #41182 .