A concrete type parameter cannot be used as `impl Trait` if `?` is used previously

[lo48576]

Code

I tried this code:

use core::fmt::Debug;

#[derive(Debug)]
pub struct Target;

#[derive(Debug)]
pub struct Source;
impl From<Source> for Target {
    fn from(_: Source) -> Self {
        Self
    }
}

fn maybe_source() -> Result<(), Source> {
    todo!()
}

pub fn typaram() -> Result<(), impl Debug> {
    maybe_source()?;
    Ok::<_, Target>(())
}

pub fn direct() -> Result<(), impl Debug> {
    maybe_source()?;
    Err(Target)
}

I expected to see this happen: Compiles successfully, with Result<(), impl Debug> instantiated as Result<(), Target>.

Instead, this happened: Compilation error.

$ cargo +nightly --version --verbose
cargo 1.60.0-nightly (c082648 2022-02-08)
release: 1.60.0-nightly
commit-hash: c082648646cbb2be266df9ecbcdc253058158d68
commit-date: 2022-02-08
host: x86_64-unknown-linux-gnu
libgit2: 1.3.0 (sys:0.13.23 vendored)
libcurl: 7.80.0-DEV (sys:0.4.51+curl-7.80.0 vendored ssl:OpenSSL/1.1.1l)
os: Linux 2.8 [64-bit]
$ cargo +nightly check
    Checking impl-type-coercion v0.1.0 (/tmp/lo48576.ed4d0c1e-416f-417d-b689-0cfe81e092af/impl-type-coercion)
error[E0308]: mismatched types
  --> src/lib.rs:20:5
   |
18 | pub fn typaram() -> Result<(), impl Debug> {
   |                                ---------- the expected opaque type
19 |     maybe_source()?;
20 |     Ok::<_, Target>(())
   |     ^^^^^^^^^^^^^^^^^^^ expected struct `Source`, found struct `Target`
   |
   = note: expected opaque type `impl Debug`
                   found struct `Target`

error[E0308]: mismatched types
  --> src/lib.rs:25:5
   |
23 | pub fn direct() -> Result<(), impl Debug> {
   |                               ---------- the expected opaque type
24 |     maybe_source()?;
25 |     Err(Target)
   |     ^^^^^^^^^^^ expected struct `Source`, found struct `Target`
   |
   = note: expected opaque type `impl Debug`
                   found struct `Target`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `impl-type-coercion` due to 2 previous errors
$

Without maybe_source()?; line, the errors don't happen.

Version it worked on

It most recently worked on:

• rustc 1.58.1 (db9d1b2 2022-01-20) (current stable)
• rustc 1.59.0-beta.6 (0426998 2022-02-02) (current beta)

Version with regression

rustc +nightly --version --verbose:

rustc 1.60.0-nightly (e646f3d2a 2022-02-10)
binary: rustc
commit-hash: e646f3d2a9541952310778288854943678738ea9
commit-date: 2022-02-10
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[lo48576]

Doctests that return Result<_, _> might be affected.

use core::fmt::Debug;

#[derive(Debug)]
pub struct Target;

#[derive(Debug)]
pub struct Source;
impl From<Source> for Target {
    fn from(_: Source) -> Self {
        Self
    }
}

/// # Examples
///
/// ```
/// use impl_type_coercion::{Target, maybe_source};
/// maybe_source()?;
/// Ok::<_, Target>(())
/// ```
pub fn maybe_source() -> Result<(), Source> {
    todo!()
}

Output of cargo +nightly test:

$ cargo +nightly test
    Finished test [unoptimized + debuginfo] target(s) in 0.00s
     Running unittests (target/debug/deps/impl_type_coercion-5c03dc1e98d7ad90)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests impl-type-coercion

running 1 test
test src/lib.rs - maybe_source (line 16) ... FAILED

failures:

---- src/lib.rs - maybe_source (line 16) stdout ----
error[E0308]: mismatched types
 --> src/lib.rs:19:1
  |
3 | fn main() { #[allow(non_snake_case)] fn _doctest_main_src_lib_rs_16_0() -> Result<(), impl core::fmt::Debug> {
  |                                                                                       --------------------- the expected opaque type
...
6 | Ok::<_, Target>(())
  | ^^^^^^^^^^^^^^^^^^^ expected struct `Source`, found struct `Target`
  |
  = note: expected opaque type `impl Debug`
                  found struct `Target`

error: aborting due to previous error

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

failures:
    src/lib.rs - maybe_source (line 16)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s

error: test failed, to rerun pass '--doc'
$

[lo48576]

searched nightlies: from nightly-2022-02-01 to nightly-2022-02-11
regressed nightly: nightly-2022-02-09
searched commit range: 734368a...0c292c9
regressed commit: e7cc3bd

bisected with cargo-bisect-rustc v0.6.1

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2022-02-01 --end=2022-02-11

[compiler-errors]

This got reverted in #93893

[lo48576]

Thank you!
Closing this issue as I confirmed this is fixed at rustc 1.60.0-nightly (e789f3a3a 2022-02-11).