ICE: Impl was matchable against Obligation(...) but now is not

[honzasp]

I got an internal compiler error when cargo testing my code, and I was unfortunately unable to reproduce on a small sample. The code is here on Github, to reproduce the error, change the literal on one line in test from 42.0 to 42.

$ rustc --version --verbose
rustc 1.0.0-nightly (522d09dfe 2015-02-19) (built 2015-02-21)
binary: rustc
commit-hash: 522d09dfecbeca1595f25ac58c6d0178bbd21d7d
commit-date: 2015-02-19
build-date: 2015-02-21
host: i686-unknown-linux-gnu
release: 1.0.0-nightly
$ RUST_BACKTRACE=1 cargo test --verbose
   Compiling spiral v0.0.1 (file:///[...]/spiral)
     Running `rustc src/main.rs --crate-name spiral --crate-type bin -g --test -C metadata=1ade813d3d860ac1 -C extra-filename=-1ade813d3d860ac1 --out-dir [...]/spiral/target --emit=dep-info,link -L dependency=[...]/spiral/target -L dependency=[...]/spiral/target/deps`
error: internal compiler error: Impl DefId { krate: 2, node: 74181 } was matchable against Obligation(predicate=Binder(TraitPredicate(core::cmp::PartialEq<_>)),depth=1) but now is not
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
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:189

stack backtrace:
   1: 0xb7231280 - sys::backtrace::write::hb58c665617e5cee2KlC
   2: 0xb725b8b0 - panicking::on_panic::hd4d4fc652390cdd8tXL
   3: 0xb7195ac0 - rt::unwind::begin_unwind_inner::h6797c7a781aac698RBL
   4: 0xb5656fd0 - rt::unwind::begin_unwind::h13117811778590822301
   5: 0xb5657800 - diagnostic::Handler::bug::h0b9167776bd8dc1aw4E
   6: 0xb61b5310 - middle::traits::select::SelectionContext<'cx, 'tcx>::rematch_impl::h7f3f2e654c4ffad9kHU
   7: 0xb61b4c30 - middle::infer::InferCtxt<'a, 'tcx>::try::h13523729267676866609
   8: 0xb619dd90 - middle::traits::select::SelectionContext<'cx, 'tcx>::confirm_candidate::ha4efb7af28b19263S8T
   9: 0xb61784f0 - middle::traits::select::SelectionContext<'cx, 'tcx>::select::h5394e35b0e7c70a6lhS
  10: 0xb6174160 - middle::traits::fulfill::FulfillmentContext<'tcx>::select::hbef8233dc9336219B0P
  11: 0xb6173ac0 - middle::traits::fulfill::FulfillmentContext<'tcx>::select_where_possible::hbd71b9b88c61613cHZP
  12: 0xb6cdddc0 - check::vtable::select_fcx_obligations_where_possible::hffd3ec74dffa624eVRb
  13: 0xb6d1a6f0 - check::FnCtxt<'a, 'tcx>::resolve_type_vars_if_possible::he1da80c3c9c1dda2rJo
  14: 0xb6cca360 - check::structurally_resolved_type::hb6535b94e7bbe37bNPt
  15: 0xb6e00220 - check::check_expr_with_unifier::h9614455191220309796
  16: 0xb6dbad90 - check::check_expr_with_unifier::check_binop::hbf8de0ab08b2ba09JMq
  17: 0xb6df3370 - check::check_expr_with_unifier::h6092115828992963581
  18: 0xb6dd9240 - check::check_expr_with_unifier::h3884107362083173097
  19: 0xb6dbad90 - check::check_expr_with_unifier::check_binop::hbf8de0ab08b2ba09JMq
  20: 0xb6df3370 - check::check_expr_with_unifier::h6092115828992963581
  21: 0xb6df3370 - check::check_expr_with_unifier::h6092115828992963581
  22: 0xb6dd9240 - check::check_expr_with_unifier::h3884107362083173097
  23: 0xb6dd0200 - check::check_expr_with_unifier::check_then_else::h483da0d6a4f3eab2MEq
  24: 0xb6decc20 - check::check_expr_with_unifier::h17365000147061483630
  25: 0xb6da6d80 - check::check_block_with_expected::hf29cd5d4f1823991zys
  26: 0xb6decc20 - check::check_expr_with_unifier::h17365000147061483630
  27: 0xb6ccafe0 - check::_match::check_match::closure.29042
  28: 0xb6ccac10 - check::_match::check_match::h77807f1365a779d3V1a
  29: 0xb6decc20 - check::check_expr_with_unifier::h17365000147061483630
  30: 0xb6da6d80 - check::check_block_with_expected::hf29cd5d4f1823991zys
  31: 0xb6df9ad0 - check::check_expr_with_unifier::h14838318111108599859
  32: 0xb6da6d80 - check::check_block_with_expected::hf29cd5d4f1823991zys
  33: 0xb6d85a80 - check::check_fn::h9733b23b5bb31869JKn
  34: 0xb6da3700 - check::check_bare_fn::hc0a904b0aabec0a4Wzn
  35: 0xb6d9aa00 - check::check_item::heaa17bb09891b8a9nTn
  36: 0xb6da19f0 - visit::walk_item::h334270929839752142
  37: 0xb6da19f0 - visit::walk_item::h334270929839752142
  38: 0xb6da19f0 - visit::walk_item::h334270929839752142
  39: 0xb6e7f750 - check_crate::closure.35867
  40: 0xb6e78b60 - check_crate::h74f1efb9c9ad0cecVjC
  41: 0xb769c480 - driver::phase_3_run_analysis_passes::ha81485be033381d8gHa
  42: 0xb7680c20 - driver::compile_input::h55e2c5089100dcc0Gba
  43: 0xb7760430 - run_compiler::h7c1df0ae32ef6a57Zbc
  44: 0xb775e810 - thunk::F.Invoke<A, R>::invoke::h4956589150906656218
  45: 0xb775d680 - rt::unwind::try::try_fn::h3108213307196454525
  46: 0xb72d5b30 - rust_try_inner
  47: 0xb72d5b00 - rust_try
  48: 0xb775d9b0 - thunk::F.Invoke<A, R>::invoke::h9387302087009601825
  49: 0xb72474c0 - sys::thread::thread_start::hb380b1da46a95e5393G
  50: 0xb2f29ea0 - start_thread
  51: 0xb7049502 - clone
  52:        0x0 - <unknown>

Could not compile `spiral`.

Caused by:
  Process didn't exit successfully: `rustc src/main.rs --crate-name spiral --crate-type bin -g --test -C metadata=1ade813d3d860ac1 -C extra-filename=-1ade813d3d860ac1 --out-dir [...]/spiral/target --emit=dep-info,link -L dependency=[...]/spiral/target -L dependency=[...]/spiral/target/deps` (status=101)

I am sorry for such a crude report, but I could not come with a better sample, as the origin of the error is a bit unclear to me.

[edwardw]

There seems to be at least four similar tickets about this "but now is not" business: this one, #19976, #18623, #18954. They could be all dups.

[tamird]

Agree with @edwardw. Either way, this does still repro with:

use std::ops::Add;

trait Scalar {}
impl Scalar for f64 {}

struct Bob;

impl<RHS: Scalar> Add <RHS> for Bob {
  type Output = Bob;
  fn add(self, rhs : RHS) -> Bob {} # this is not even valid
}

fn main() {
  let b = Bob + 3.5;
  // Internal compiler error (should be type error?):
  b + 3
}

[TimNN]

I think #23825 is also a dupe of this.

The following code reproduces the same issue:

fn main() {
    let a: f32 = 0f32 * 0.0;
    a * 0;
}

Note that even though a is explicitly typed, this still causes the ICE.

[arielb1]

Duplicate of #24352