ICE with specialization

[gnzlbg]

The following example ices the compiler:

#![feature(specialization)]

trait A : Sized {
    fn foo(self, _: Self) -> Self { self }
}

impl A for u8 {}
impl A for u16 {}

// requires specialization:
impl FromA<u8> for u16 {
    fn from(x: u8) -> u16 { x as u16 }
}

trait FromA<T> {
    fn from(T) -> Self;
}

//impl<T: A, U: A > FromA<T> for U { // no ICE
impl<T: A, U: A + FromA<T>> FromA<T> for U {  // ICE
    default fn from(x: T) -> Self { // specialization: :(
        ToA::to(x)
    }
}

trait ToA<T> {
    fn to(self) -> T;
}

// From implies Into
impl<T, U> ToA<U> for T
    where U: FromA<T>
{
    fn to(self) -> U {
        U::from(self)
    }
}

#[allow(dead_code)]
fn foo<T: A, U: A>(x: T, y: U) -> U {
    x.foo(y.to()).to()
}

fn main() {
    
    let z = foo(8u8, 1u16);
}

[eminence]

Bug triage: On current nightly (rustc 1.23.0-nightly (e97ba8328 2017-11-25)) this is no longer an ICE, instead it yields the following error:

error[E0275]: overflow evaluating the requirement `T: FromA<U>`
  --> a.rs:41:13
   |
41 |     x.foo(y.to()).to()
   |             ^^
   |
   = note: required because of the requirements on the impl of `FromA<U>` for `T`
   = note: required because of the requirements on the impl of `ToA<T>` for `U`

error: aborting due to previous error

[estebank]

This can be closed by adding the repro case as a regression test.