ICE: When moving Fn bound to `where` clause

[japaric]

STR

#![feature(unboxed_closures)]

struct Bencher;

// ICE
fn warm_up<'a, F>(f: F)
    where F: |&: &'a mut Bencher|
{
    unimplemented!();
}

// OK
//fn warm_up<'a, F: |&: &'a mut Bencher|>(f: F) { unimplemented!() }

fn main() {
    // ICE trigger
    warm_up(|&: b: &mut Bencher| unimplemented!() );

    // OK
    warm_up(|&: b| unimplemented!() );
}

Backtrace

ice.rs:17:5: 17:12 error: internal compiler error: cannot relate bound region: ReLateBound(7, BrNamed(syntax::ast::DefId{krate: 0u32, node: 14u32}, 'a)) <= ReInfer(8)
ice.rs:17     warm_up(|&: b: &mut Bencher| unimplemented!() );
              ^~~~~~~
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>', /var/tmp/paludis/build/dev-lang-rust-scm/work/rust-scm/src/libsyntax/ast_util.rs:791

stack backtrace:
   1:     0x7f7d7ee16ec0 - rt::backtrace::imp::write::h0d672ecc305d940eAxr
   2:     0x7f7d7ee19f90 - <unknown>
   3:     0x7f7d7f5f0c80 - unwind::begin_unwind_inner::had82a5715f0c70a6Zhe
   4:     0x7f7d7b500150 - <unknown>
   5:     0x7f7d7b5000a0 - diagnostic::SpanHandler::span_bug::h5311365fdefe54613uF
   6:     0x7f7d7fba1610 - driver::session::Session::span_bug::hdc7a4659699633a7bOA
   7:     0x7f7d8007a7c0 - middle::typeck::infer::region_inference::RegionVarBindings<'a>::make_subregion::heb4b085f2458b2f3lX5
   8:     0x7f7d800609f0 - middle::typeck::infer::sub::Sub<'f>.Combine::regions::h2742a65556179ddajs8
   9:     0x7f7d80060d60 - middle::typeck::infer::sub::Sub<'f>.Combine::contraregions::h661a459f8cc4d8c0Jr8
  10:     0x7f7d8004a710 - middle::typeck::infer::sub::Sub<'f>.Combine::tys::he592ec9eb9d995dfJA8
  11:     0x7f7d8008f850 - <unknown>
  12:     0x7f7d80060f70 - <unknown>
  13:     0x7f7d8004a710 - middle::typeck::infer::sub::Sub<'f>.Combine::tys::he592ec9eb9d995dfJA8
  14:     0x7f7d8008f150 - <unknown>
  15:     0x7f7d80047d60 - <unknown>
  16:     0x7f7d8008efd0 - <unknown>
  17:     0x7f7d8008d3a0 - <unknown>
  18:     0x7f7d800b9e70 - <unknown>
  19:     0x7f7d800b9ba0 - <unknown>
  20:     0x7f7d800b99f0 - <unknown>
  21:     0x7f7d7ff3d0a0 - middle::typeck::infer::mk_sub_trait_refs::h215f9c26284f866anxd
  22:     0x7f7d7ff3ca10 - <unknown>
  23:     0x7f7d7ff3dd40 - <unknown>
  24:     0x7f7d7ff3ae60 - <unknown>
  25:     0x7f7d7fefff00 - middle::ty::each_bound_trait_and_supertraits::h1becbefbed517889NOI
  26:     0x7f7d7ff3a8e0 - <unknown>
  27:     0x7f7d7ff38720 - <unknown>
  28:     0x7f7d7ff48340 - <unknown>
  29:     0x7f7d7ff481b0 - middle::typeck::check::FnCtxt<'a>::opt_node_ty_substs::hf2958efb0b7310b21XU
  30:     0x7f7d7ff44660 - middle::typeck::check::vtable::early_resolve_expr::hac1019c39f97238axnM
  31:     0x7f7d7ff4b960 - <unknown>
  32:     0x7f7d7ff4d2f0 - middle::typeck::check::vtable::resolve_in_block::ha70b06cbd736c562H7M
  33:     0x7f7d7ffa9e60 - <unknown>
  34:     0x7f7d7ffa2fd0 - middle::typeck::check::check_item::hd430fed06beea5592BT
  35:     0x7f7d7ffa9c60 - middle::typeck::check::check_item_types::h06acd5dca8e5d103b2S
  36:     0x7f7d7f9c3330 - <unknown>
  37:     0x7f7d801b8210 - middle::typeck::check_crate::hcdc2f87496d7d542HFk
  38:     0x7f7d802854f0 - driver::driver::phase_3_run_analysis_passes::haa0c1431e392c3b2Vfz
  39:     0x7f7d80280630 - driver::driver::compile_input::h0a11de02699c1d9111y
  40:     0x7f7d8032d4b0 - <unknown>
  41:     0x7f7d8032d3c0 - <unknown>
  42:     0x7f7d8033fdf0 - <unknown>
  43:     0x7f7d8033fbf0 - <unknown>
  44:     0x7f7d80d8e840 - <unknown>
  45:     0x7f7d7f640990 - <unknown>
  46:     0x7f7d7f640980 - rust_try
  47:     0x7f7d7f5ee2e0 - unwind::try::hbae38345f951cdabf6d
  48:     0x7f7d7f5ee080 - task::Task::run::hfc294062f8a07e89xcd
  49:     0x7f7d80d8e600 - <unknown>
  50:     0x7f7d7f5efec0 - <unknown>
  51:     0x7f7d7e8a7000 - start_thread
  52:     0x7f7d7f2bf269 - clone
  53:                0x0 - <unknown>

Version

rustc 0.12.0-pre (78ec3904b 2014-08-16 13:01:09 +0000)

cc @pcwalton