Compilation error regression on nightly

[sunfishcode]

I tried this code (reduced testcase from code in cap-std):

use std::path::Path;
use std::ffi::OsStr;
use std::ops::Deref;

fn frob(path: &str) -> impl Deref<Target = Path> + '_ {
    OsStr::new(path).as_ref()
}

fn open_parent<'path>(_path: &'path Path) {
    todo!()
}

fn main() {
    let old_path = frob("hello");

    open_parent(&old_path);
}

When compiled with stable Rust, it compiles with no errors.

When compiled with nightly Rust, it gets the following error:

error[E0308]: mismatched types
  --> src/main.rs:16:17
   |
5  | fn frob(path: &str) -> impl Deref<Target = Path> + '_ {
   |                        ------------------------------ the found opaque type
...
16 |     open_parent(&old_path);
   |                 ^^^^^^^^^ expected struct `Path`, found opaque type
   |
   = note:   expected struct `Path`
           found opaque type `impl Deref<Target = Path>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `abc` due to previous error

The error can be avoided by changing &old_path to &*old_path, which works on both stable and nightly. I've already fixed the upstream code do to this; I'm filing this bug as it may affect other users as well.

Meta

Stable rustc --version --verbose:

rustc 1.58.1 (db9d1b20b 2022-01-20)
binary: rustc
commit-hash: db9d1b20bba1968c1ec1fc49616d4742c1725b4b
commit-date: 2022-01-20
host: x86_64-unknown-linux-gnu
release: 1.58.1
LLVM version: 13.0.0

Nightly rustc --version --verbose:

rustc 1.60.0-nightly (0c292c966 2022-02-08)
binary: rustc
commit-hash: 0c292c9667f1b202a9150d58bdd2e89e3e803996
commit-date: 2022-02-08
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

[oli-obk]

Ah interesting. Treating any equality check between opaque types and other types as "will succeed modulo the opaque type obligation" will skip other code paths like trying autoderef

[Emilgardis]

This also fails rustdoc disambiguator with a boxed dyn Error

//! ```rust
//! use std::fmt::Write;
//! let mut s = String::new();
//! write!(s, "Hello")?;
//! # Ok::<(), std::boxed::Box<dyn std::error::Error + 'static>>(())
//! ```

cargo test --doc
   Compiling rust-doc v0.1.0 (G:\workspace\rust-doc)
    Finished test [unoptimized + debuginfo] target(s) in 7.25s
   Doc-tests rust-doc

running 1 test
test src\lib.rs - (line 1) ... FAILED

failures:

---- src\lib.rs - (line 1) stdout ----
error[E0308]: mismatched types
 --> src\lib.rs:5:1
  |
2 | fn main() { #[allow(non_snake_case)] fn _doctest_main_src_lib_rs_1_0() -> Result<(), impl core::fmt::Debug> {
  |                                                                                      --------------------- the expected opaque type
...
6 | Ok::<(), std::boxed::Box<dyn std::error::Error + 'static>>(())
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::fmt::Error`, found struct `Box`
  |
  = note: expected opaque type `impl Debug`
                  found struct `Box<dyn std::error::Error>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
Couldn't compile the test.

bisected
searched nightlies: from nightly-2022-02-08 to nightly-2022-02-10
regressed nightly: nightly-2022-02-09
searched commit range: 734368a...0c292c9
regressed commit: e7cc3bd