Internal compiler error anonymous bound region BrAnon(0) in binding but not trait ref

[SeeSpring]

The following code causes an error

impl<'b, B: Ty<'b>, I: SIterator, F> SIterator for FilterMap<I, F> where
    F: FnMut(<I as Ty>::V) -> Option<<B as Ty>::V>
{}

pub trait SIterator: for<'a> Ty<'a> {}

pub trait Ty<'a> {
    type V;
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0465]: multiple rlib candidates for `debug_unreachable` found
  |
  = note: candidate #1: /playground/target/debug/deps/libdebug_unreachable-53cbbc8451f8a571.rlib
  = note: candidate #2: /playground/target/debug/deps/libdebug_unreachable-9f86ee3a7179ede7.rlib

error[E0412]: cannot find type `FilterMap` in this scope
 --> src/lib.rs:1:52
  |
1 | impl<'b, B: Ty<'b>, I: SIterator, F> SIterator for FilterMap<I, F> where
  |                                                    ^^^^^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
  |
1 | use core::iter::FilterMap;
  |
1 | use futures::stream::FilterMap;
  |
1 | use itertools::__std_iter::FilterMap;
  |
1 | use rayon::iter::FilterMap;
  |
and 2 other candidates

error: internal compiler error: src/librustc_typeck/astconv.rs:1097: anonymous bound region BrAnon(0) in binding but not trait ref
 --> src/lib.rs:2:31
  |
2 |     F: FnMut(<I as Ty>::V) -> Option<<B as Ty>::V>
  |                               ^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:578:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0412`.

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 1.37.0-nightly (b25ee6449 2019-06-17) running on x86_64-unknown-linux-gnu

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

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

error: Could not compile `playground`.

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

[Centril]

Reduced:

pub trait SIterator {}

pub trait Ty<'a> {
    type V;
}

struct FilterMap<F>(F);

impl<X, F> SIterator for FilterMap<F>
where
    F: FnOnce(<X as Ty<'_>>::V) -> Option<<X as Ty<'_>>::V>
{}

[pnkfelix]

seems like this bug dates at least to when '_ was stabilized in 1.26.0

[pnkfelix]

bug dates back to at least nightly-2017-10-15 and perhaps earlier. It seems reasonable right now to assume its been present ever since feature(underscore_lifetimes) was introduced.

[pnkfelix]

I'm going to have to do some background reading to remember what we are supposed to do in a case like this.

If you remove the occurrences of '_ you get the same ICE behavior, and that behavior dates all the way back to rust 1.10.0:

% rustc +1.10.0 issue-62200.rs
issue-62200.rs:11:32: 11:52 error: internal compiler error: ../src/librustc_typeck/astconv.rs:916: anonymous bound region BrAnon(0) in binding but not trait ref
issue-62200.rs:11     F: FnOnce(<X as Ty>::V) -> Option<<X as Ty>::V>
                                                 ^~~~~~~~~~~~~~~~~~~~

[pnkfelix]

triage: P-medium. Removing nomination. Assigning to self.

[smmalis37]

I just ran into this on 1.41.1. Attempted to minimize what I was doing and ended up basically identical to Centril's above.

struct S {}

trait T<'a>{
    type A;
}

impl T<'_> for S {
    type A = u32;
}

fn foo(x: impl Fn(<S as T>::A) -> <S as T>::A) {
}