Overlap checking with downstream impl is insufficient w.r.t. fundamental types; causes ICE

[qnighy]

Overlap checker considers possible downstream impl, but it is insufficient if it may contain fundamental types. May cause ICE.

#![crate_type = "lib"]
#![crate_name = "foo"]

pub trait Trait1<X> {
    type Output;
}
pub trait Trait2<X> {}

impl<X, T> Trait1<X> for T where T: Trait2<X> {
    type Output = ();
}
impl<X> Trait1<Box<X>> for A {
    type Output = i32;
}

pub struct A;

fn f<X, T: Trait1<Box<X>>>() {
    println!("k: {}", ::std::mem::size_of::<<T as Trait1<Box<X>>>::Output>());
}

pub fn g<X, T: Trait2<Box<X>>>() {
    f::<X, T>();
}

extern crate foo;

use foo::{Trait2, A, g};

struct B;

impl Trait2<Box<B>> for A {}

fn main() {
    g::<B, A>();
}

In stable (1.18.0), ICE is triggered when compiling downstream main.

$ rustc --version
rustc 1.18.0 (03fc9d622 2017-06-06)
$ cargo build
   Compiling foo v0.1.0 (file:///C:/Users/qnighy/workdir/orphan-test/foo)
   Compiling orphan-test v0.1.0 (file:///C:/Users/qnighy/workdir/orphan-test)
error: internal compiler error: src\librustc\infer\mod.rs:715: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <foo::A as foo::Trait1<std::boxed::Box<B>>>, item_name: Output(445) }, _)),depth=1),Ambiguity)]` resolving bounds after type-checking

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:375
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `orphan-test`.

To learn more, run the command again with --verbose.

In nightly, ICE is triggered when compiling even upstream foo. Perhaps this is a different bug.

$ rustc +nightly --version
rustc 1.20.0-nightly (582af6e1a 2017-07-19)
$ cargo +nightly build
   Compiling foo v0.1.0 (file:///C:/Users/qnighy/workdir/orphan-test/foo)
error: internal compiler error: src\librustc\infer\mod.rs:573: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<T as std::marker::Sized>)),depth=1),Unimplemented), FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<T as Trait2<std::boxed::Box<X>>>)),depth=1),Unimplemented)]` resolving bounds after type-checking

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.20.0-nightly (582af6e1a 2017-07-19) running on x86_64-pc-windows-msvc

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:437:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `foo`.

To learn more, run the command again with --verbose.

[kennytm]

The nightly ICE should be a different bug, related to size_of of an associated type.

trait Trait1 {
    type Output;
}
fn f<T: Trait1>() {
    ::std::mem::size_of::<T::Output>();
}
fn main() {}

[arielb1]

Yeah I always hated that check. So now we see it is insufficient. Let's hope we can fix this without nuking the entire ecosystem.