FP unreachable-pub, breaks the code

[matthiaskrgr]

Given the following code:

#![feature(auto_traits)]
#![feature(negative_impls)]

pub trait PubPrincipal {}
auto trait PrivNonPrincipal {}

pub fn leak_dyn_nonprincipal() -> Box<dyn PubPrincipal + PrivNonPrincipal> { loop {} }
//~^ WARN private trait `PrivNonPrincipal` in public interface
//~| WARN this was previously accepted

#[deny(missing_docs)]
fn container() {
    impl dyn PubPrincipal {
         fn check_doc_lint() {} //~ ERROR missing documentation for an associated function
    }
    impl dyn PubPrincipal + PrivNonPrincipal {
        pub fn check_doc_lint() {} // OK, no missing doc lint
    }
}

fn main() {}

The suggestion breaks the build:

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0445]: private trait `PrivNonPrincipal` in public interface
 --> src/main.rs:7:1
  |
5 | auto trait PrivNonPrincipal {}
  | --------------------------- `PrivNonPrincipal` declared as private
6 |
7 | pub fn leak_dyn_nonprincipal() -> Box<dyn PubPrincipal + PrivNonPrincipal> { loop {} }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak private trait

error: aborting due to previous error

For more information about this error, try `rustc --explain E0445`.
Original diagnostics will follow.

warning: unreachable `pub` item
  --> src/main.rs:17:9
   |
17 |         pub fn check_doc_lint() {} // OK, no missing doc lint
   |         ---^^^^^^^^^^^^^^^^^^^^
   |         |
   |         help: consider restricting its visibility: `pub(crate)`
   |
   = note: requested on the command line with `--force-warn unreachable-pub`