Compiler crash with new default parameters

[flying-sheep]

When trying to use this feature in japaric-archived/complex.rs#9, the following happened:

   Compiling complex v0.0.0 (file:///home/phil/Dev/rust/_foreign_projects/complex.rs)
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:149:1: 156:2 error: internal compiler error: impl `VtableImpl(impl_def_id=DefId { krate: 0, node: 1681 }:Complex<T>.Neg, substs=Substs[types=[[_];[];[]], regions=[[];[];[]]], nested=[[Obligation(predicate=Binder(TraitPredicate(TraitRef(T, core::kinds::Sized))),depth=1), Obligation(predicate=Binder(TraitPredicate(TraitRef(T, core::ops::Neg))),depth=1)];[];[]])` did not contain projection for `Obligation(predicate=<Complex<T> as TraitRef(Complex<T>, core::ops::Neg)>::Output,depth=0)`
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:149 impl<T> Neg for Complex<T> where T: Neg {
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:150     fn neg(self) -> Complex<T> {
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:151         Complex {
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:152             re: -self.re,
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:153             im: -self.im,
/home/phil/Dev/rust/_foreign_projects/complex.rs/src/lib.rs:154         }
                                                                ...
note: the compiler unexpectedly panicked. 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
thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:123

stack backtrace:
   1:     0x7f6d403552d0 - sys::backtrace::write::h8532e701ef86014f4it
   2:     0x7f6d4037ab00 - failure::on_fail::h7532e1f79d134d5dzvz
   3:     0x7f6d402e01c0 - rt::unwind::begin_unwind_inner::h97b151606151d62deaz
   4:     0x7f6d3b1b4c60 - rt::unwind::begin_unwind::h15809447133099964284
   5:     0x7f6d3b1b4bf0 - diagnostic::SpanHandler::span_bug::he8142ababcc30c39DFF
   6:     0x7f6d3e6b3e70 - middle::traits::project::project_type::h947eece142ef049d52P
   7:     0x7f6d3e6b0fd0 - middle::traits::project::opt_normalize_projection_type::hb3defb9cc9365d1e8UP
   8:     0x7f6d3e69eba0 - middle::traits::project::normalize_projection_type::hdc293893275ee559JTP
   9:     0x7f6d3e6b2470 - middle::traits::project::AssociatedTypeNormalizer<'a, 'b, 'tcx>.TypeFolder<'tcx>::fold_ty::h801cbd2cdff2eff1kSP
  10:     0x7f6d3f96f2e0 - middle::ty_fold::Rc<T>.TypeFoldable<'tcx>::fold_with::h6325524173844043840
  11:     0x7f6d3f96fe90 - middle::ty_fold::VecPerParamSpace<T>.TypeFoldable<'tcx>::fold_with::h13884369302522804796
  12:     0x7f6d3f98b280 - check::FnCtxt<'a, 'tcx>::instantiate_bounds::hed550a9659b70335Oll
  13:     0x7f6d3f9a3fc0 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>::check_impl::closure.29997
  14:     0x7f6d3f9a0430 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>::with_fcx::hb1283961ed8977b7Gfi
  15:     0x7f6d3f9a6d70 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>.Visitor<'v>::visit_item::h001ababd87597e37soi
  16:     0x7f6d3fb93fe0 - check_crate::unboxed_closure.40162
  17:     0x7f6d3fb8ec30 - check_crate::h19fb6dea5733566ajsx
  18:     0x7f6d408a6640 - driver::phase_3_run_analysis_passes::h46b1604d9f9f5633Tva
  19:     0x7f6d40894ae0 - driver::compile_input::h68b8602933aad8d7wba
  20:     0x7f6d4095feb0 - thunk::F.Invoke<A, R>::invoke::h18029802347644288836
  21:     0x7f6d4095ec60 - rt::unwind::try::try_fn::h6518866316425934196
  22:     0x7f6d403e1400 - rust_try_inner
  23:     0x7f6d403e13f0 - rust_try
  24:     0x7f6d4095efb0 - thunk::F.Invoke<A, R>::invoke::h15513809553472565307
  25:     0x7f6d40366e40 - sys::thread::thread_start::h5ea7ba97235331d5a9v
  26:     0x7f6d3a9d9250 - start_thread
  27:     0x7f6d3ff97219 - clone
  28:                0x0 - <unknown>

Could not compile `complex`.

To learn more, run the command again with --verbose.

[japaric]

Minimal script:

#![crate_type = "lib"]

use std::ops::Neg;

struct Complex<T>;

impl<T> Neg for Complex<T> where T: Neg {
    fn neg(self) -> Complex<T> {
        loop {}
    }
}

Tested with: rustc 0.13.0-nightly (c6c786671 2015-01-04 00:50:59 +0000)

cc @nikomatsakis (I don't know if you have already fixed this.)

---

@flying-sheep You are missing the associated type in the impl Neg and also you need to bound the assoc type in where T: Neg.

This should work:

#![crate_type = "lib"]
#![feature(associated_types)]

use std::ops::Neg;

struct Complex<T>;

impl<T> Neg for Complex<T> where T: Neg<Output=T> {
    type Output = Complex<T>;

    fn neg(self) -> Complex<T> {
        loop {}
    }
}

[flying-sheep]

great, thanks! i just figured out the first part myself, but good thing i found this compiler bug!

and you mean that i need where T: Neg<Output=T>, as there already is where T: Neg…?

[huonw]

Closing as a dupe of #20347, thanks for filing.