src/test/ui/issues/issue-47139-1.rs

// run-pass
// Regression test for issue #47139:
//
// Coherence was encountering an (unnecessary) overflow trying to
// decide if the two impls of dummy overlap.
//
// The overflow went something like:
//
// - `&'a ?T: Insertable` ?
// - let ?T = Option<?U> ?
// - `Option<?U>: Insertable` ?
// - `Option<&'a ?U>: Insertable` ?
// - `&'a ?U: Insertable` ?
//
// While somewhere in the middle, a projection would occur, which
// broke cycle detection.
//
// It turned out that this cycle was being kicked off due to some
// extended diagnostic attempts in coherence, so removing those
// sidestepped the issue for now.

#![allow(dead_code)]

pub trait Insertable {
    type Values;

    fn values(self) -> Self::Values;
}

impl<T> Insertable for Option<T>
    where
    T: Insertable,
    T::Values: Default,
{
    type Values = T::Values;

    fn values(self) -> Self::Values {
        self.map(Insertable::values).unwrap_or_default()
    }
}

impl<'a, T> Insertable for &'a Option<T>
    where
    Option<&'a T>: Insertable,
{
    type Values = <Option<&'a T> as Insertable>::Values;

    fn values(self) -> Self::Values {
        self.as_ref().values()
    }
}

impl<'a, T> Insertable for &'a [T]
{
    type Values = Self;

    fn values(self) -> Self::Values {
        self
    }
}

trait Unimplemented { }

trait Dummy { }

struct Foo<T> { t: T }

impl<'a, U> Dummy for Foo<&'a U>
    where &'a U: Insertable
{
}

impl<T> Dummy for T
    where T: Unimplemented
{ }

fn main() {
}