Failed to find candidates after PathProbeImplTrait

[CohenArthur]

Summary

We get zero candidates after a probing for a trait function call

Reproducer

I tried this code:

#[lang = "sized"]
trait Sized {}

enum Result {
    Ok(i32),
    Err(i32)
}

pub trait From<T>: Sized {
    /// Performs the conversion.
    fn from(_: T) -> Self;
}

impl<T> From<T> for T {
    fn from(t: T) -> Self { t }
}

pub trait Try {
    /// The type of this value when viewed as successful.
    // #[unstable(feature = "try_trait", issue = "42327")]
    // type Ok;
    /// The type of this value when viewed as failed.
    // #[unstable(feature = "try_trait", issue = "42327")]
    // type Error;

    /// Applies the "?" operator. A return of `Ok(t)` means that the
    /// execution should continue normally, and the result of `?` is the
    /// value `t`. A return of `Err(e)` means that execution should branch
    /// to the innermost enclosing `catch`, or return from the function.
    ///
    /// If an `Err(e)` result is returned, the value `e` will be "wrapped"
    /// in the return type of the enclosing scope (which must itself implement
    /// `Try`). Specifically, the value `X::from_error(From::from(e))`
    /// is returned, where `X` is the return type of the enclosing function.
    fn into_result(self) -> Result;

    /// Wrap an error value to construct the composite result. For example,
    /// `Result::Err(x)` and `Result::from_error(x)` are equivalent.
    fn from_error(v: i32) -> Self;

    /// Wrap an OK value to construct the composite result. For example,
    /// `Result::Ok(x)` and `Result::from_ok(x)` are equivalent.
    fn from_ok(v: i32) -> Self;
}

impl Try for Result {
    // type Ok = i32;
    // type Error = i32;

    fn into_result(self) -> Result {
        self
    }

    fn from_ok(v: i32) -> Self {
        Result::Ok(v)
    }

    fn from_error(v: i32) -> Self {
        Result::Err(v)
    }
}

fn print(s: &str, value: i32) {
    extern "C" { fn printf(s: *const i8, ...); }

    unsafe { printf(s as *const str as *const i8, value); }
}

fn bar() -> Result {
    Result::Ok(15)
}

fn baz() -> Result {
    Result::Err(15)
}

fn foo() -> Result {
    let b = match baz() {
        Result::Ok(value) => value,
        Result::Err(err) => { return Try::from_error(From::from(err)); },
    };

    Result::Ok(15 + b)
}

fn main() {
    foo();
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=66219ed4855ae9e6346c285e2f93f506

Actual behavior

The following assertion is triggered:

	  auto candidates
	    = Resolver::PathProbeImplTrait::Probe (receiver, final_segment,
						   trait_ref);
	  if (candidates.size () == 0)
	    {
	      // this means we are defaulting back to the trait_item if
	      // possible
	      Resolver::TraitItemReference *trait_item_ref = nullptr;
	      bool ok = trait_ref->lookup_hir_trait_item (*trait_item.value (),
							  &trait_item_ref);
	      rust_assert (ok);				    // found
	      rust_assert (trait_item_ref->is_optional ()); // has definition

              // ^ HERE

so since we didn't find any candidates, we assume that there must be a default implementation for the function in the trait definition itself - but we should actually typecheck this to the definition of from_error in the impl of Try by Result.

This doesn't cause any issues with rustc

Expected behavior

No error

GCC Version

latest master

[CohenArthur]

Note that using the expected type works as expected:

<Result as Try>::from_error(From::from(err))