Type inference can pick private-inaccessible types

I tried this code:

//! `dep`

pub trait Trait<CrateLocalParam> {
    #[doc(hidden)]
    fn __do_not_impl_manually();
}

pub fn assert_impls_trait_locally<T, CrateLocalParam>()
where
    T: Trait<CrateLocalParam>,
{}

#[macro_export]
macro_rules! crate_local_impl_Trait {( for $T:ty ) => (
    const _: () = {
        pub(crate) enum CurrentCrate {}

        impl $crate::Trait<CurrentCrate> for $T {
            fn __do_not_impl_manually() {}
        }
    };
)}

crate_local_impl_Trait!(for u8); // A

fn _demo() {
    assert_impls_trait_locally::<u8, _>(); // OK
}

alongside some ::dep-dependent:

use ::dep::*;

crate_local_impl_Trait!(for u8); // B

fn _demo2() {
    // Error, **multiple** impls found !???
    assert_impls_trait_locally::<u8, _>(); // ☹️
}

I expected to see this happen: In the context of _demo2, only its crate-local CurrentCrate is in scope. Thus, no inference ambiguity error should ensue.

Instead, this happened:

• Not only does a type inference ambiguity error occur,

  error[E0283]: type annotations needed
    --> src/lib.rs:21:9
     |
  21 |         assert_impls_some_trait_locally::<u8, _>(); // ☹️
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `CrateLocalParam` declared on the function `assert_impls_some_trait_locally`
     |
  note: multiple `impl`s satisfying `u8: Trait<_>` found
    --> src/lib.rs:17:5
     |
  17 |     crate_local_impl_Trait!(for u8); // B
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: and another `impl` found in the `dep` crate: `impl Trait<dep::_::CurrentCrate> for u8;`
  note: required by a bound in `dep::assert_impls_some_trait_locally`
    --> dep/src/lib.rs:10:12
     |
   8 |     pub fn assert_impls_some_trait_locally<T, CrateLocalParam>()
     |            ------------------------------- required by a bound in this function
   9 |     where
  10 |         T: Trait<CrateLocalParam>,
     |            ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impls_some_trait_locally`
     = note: this error originates in the macro `crate_local_impl_Trait` (in Nightly builds, run with -Z macro-backtrace for more info)
  

• but if we remove our own local crate_local_impl_Trait!(for u8); // B invocation, type inference will actively try to pick ::dep::CurrentCrate for CurrentCrate (rather than saying that no types could be found), resulting in a type privacy error.

      error: type `dep::_::CurrentCrate` is private
        --> src/lib.rs:21:9
         |
      21 |         assert_impls_some_trait_locally::<u8, _>(); // ☹️
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ private type
  

Meta

Checked on stable and nightly:

rustc --version --verbose:

1.94.0-nightly (2026-01-13 2850ca8295bc253186b2)

Start by reproducing both Rust examples from the issue with rustc: the case with local and dependency impls, and the case with only the dependency impl. Trace type inference and privacy handling around the generic _ parameter. Done means inaccessible dependency types are not selected and the local invocation does not produce an inference ambiguity.