ICE with type_alias_impl_trait: errors selecting obligation during MIR typeck

[Nemo157]

#![feature(type_alias_impl_trait)]

pub trait Foo {}

pub trait Bar {
    type Foo: Foo;

    fn foo() -> Self::Foo;
}

pub trait Baz {
    type Foo: Foo;
    type Bar: Bar<Foo = Self::Foo>;

    fn foo() -> Self::Foo;
    fn bar() -> Self::Bar;
}

impl Foo for () {}

impl Bar for () {
    type Foo = FooImpl;

    fn foo() -> Self::Foo {
        ()
    }
}

pub type FooImpl = impl Foo;
pub type BarImpl = impl Bar<Foo = FooImpl>;

impl Baz for () {
    type Foo = FooImpl;
    type Bar = BarImpl;

    fn foo() -> Self::Foo {
        ()
    }

    fn bar() -> Self::Bar {
        ()
    }
}

(playground)

error: internal compiler error: errors selecting obligation during MIR typeck: [FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: [()], item_def_id: DefId(0:14 ~ playground[6e48]::Bar[0]::Foo[0]) }, ())), depth=0),MismatchedProjectionTypes(Sorts(ExpectedFound { expected: FooImpl, found: () })))]

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:361:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.heygears.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.heygears.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
   8: std::panicking::begin_panic
   9: <rustc_errors::Handler as core::ops::drop::Drop>::drop
  10: core::ptr::real_drop_in_place
  11: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  12: core::ptr::real_drop_in_place
  13: rustc_interface::interface::run_compiler_in_existing_thread_pool
  14: std::thread::local::LocalKey<T>::with
  15: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack

changing type Foo = FooImpl; into type Foo = (); inside impl Bar for () will successfully compile

[Nemo157]

@rustbot modify labels: A-impl-trait, F-type_alias_impl_trait, C-bug, I-ICE, T-compiler, requires-nightly

[chrissimpkins]

ICE

Source: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=a68dbb4a4b78a06788f6f148c459f4cf

Stack trace

 Compiling playground v0.0.1 (/playground)
error: internal compiler error: errors selecting obligation during MIR typeck: [FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: [()], item_def_id: DefId(0:5 ~ playground[fd83]::Bar[0]::Foo[0]) }, ())), depth=0),MismatchedProjectionTypes(Sorts(ExpectedFound { expected: FooImpl, found: () })))]

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:357:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.heygears.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.heygears.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: std::panicking::begin_panic
  13: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
  14: core::ptr::drop_in_place
  15: core::ptr::drop_in_place
  16: core::ptr::drop_in_place
  17: rustc_interface::interface::run_compiler_in_existing_thread_pool
  18: scoped_tls::ScopedKey<T>::set
  19: syntax::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

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: rustc 1.43.0-nightly (07a34df18 2020-02-08) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `playground`.

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

Cargo bisection

cargo bisect-rustc --start 2019-08-03 --end 2019-08-04 --prompt

regression in nightly-2019-08-04

regression in nightly-2019-08-04
fetching https://static.rust-lang.org/dist/2019-08-04/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2019-08-04: 40 B / 40 B [=====================================================] 100.00 % 279.46 KB/s converted 2019-08-04 to 6e0d27d9368e2982bef8e1c4ac14d622c5ad018e
fetching https://static.rust-lang.org/dist/2019-08-03/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2019-08-03: 40 B / 40 B [=====================================================] 100.00 % 447.32 KB/s converted 2019-08-03 to b0e40bfba44836ad30051ffb077c1cfc5bf4a59f
looking for regression commit between 2019-08-04 and 2019-08-03
fetching commits from b0e40bfba44836ad30051ffb077c1cfc5bf4a59f to 6e0d27d9368e2982bef8e1c4ac14d622c5ad018e
opening existing repository at "rust.git"
refreshing repository
looking up first commit
looking up second commit
checking that commits are by bors and thus have ci artifacts...
finding bors merge commits
found 7 bors merge commits in the specified range
opening existing repository at "rust.git"
refreshing repository
no commits between b0e40bfba44836ad30051ffb077c1cfc5bf4a59f and 6e0d27d9368e2982bef8e1c4ac14d622c5ad018e within last 167 days

git log --oneline --author=bors b0e40bfba44836ad30051ffb077c1cfc5bf4a59f..6e0d27d9368e2982bef8e1c4ac14d622c5ad018e

6e0d27d Auto merge of #63059 - Centril:sound-bind-by-move, r=matthewjasper
452087b Auto merge of #63242 - pietroalbini:move-azure-pipelines, r=Mark-Simulacrum
a457433 Auto merge of #63234 - Centril:rollup-h9t731z, r=Centril
8e917f4 Auto merge of #62946 - RalfJung:miri_type_dispatch_first, r=oli-obk
d727071 Auto merge of #63180 - varkor:trait-alias-impl-trait, r=Centril
d9bd4b2 Auto merge of #63228 - Centril:rollup-x39p5ga, r=Centril

[JohnTitor]

Triage: This is no longer ICE with the latest nightly, marking as E-needs-test.

[Nemo157]

Though the error seems incorrect, in a slightly less opaque playground I don't see where it is getting the obligation <B as Bar>::Foo == A from, nowhere does the code assert that is true.

[nikomatsakis]

This example should be disallowed with min_type_alias_impl_trait but is not. I filed #86731 to cover that.

[nikomatsakis]

I agree with @Nemo157 that the behavior here is a bug, I don't think we handle impl Trait appearing in these sorts of contexts very well, at least within the defining module.

[JohnTitor]

Thanks for triaging, we still should add a regression test to prevent ICE at least, right?

[nikomatsakis]

Yep, that's good