opaque type needlessly inferred to be recursive

[aliemjay]

The following code should compile. However we infer two conflicting hidden types, Opaque<A> := Opaque<A> and Opaque<B> := u8, causing an error:

#![feature(type_alias_impl_trait)]
type Opaque<T> = impl Sized;
fn test<A, B>(
    arg: Vec<(Opaque<A>, u8)>,
) -> impl IntoIterator<Item = (Opaque<A>, Opaque<B>)> {
    arg
    //~^ ERROR concrete type differs from previous defining opaque type use
    //~| expected `Opaque<T>`, got `u8`
}

Another case to show that it is not enough to naively ignore the recursive definition, as it may have different arguments (Opaque<'a, 'b> := Opaque<'static, 'static>).

#![feature(type_alias_impl_trait)]

type Opaque<'a, 'b> = impl Sized + 'a + 'b;
//~^ ERROR concrete type differs from previous defining opaque type use
//~| expected `Opaque<'static, 'static>`, got `()`

// `Opaque<'a, 'b> := ()`
fn get_one<'a, 'b>() -> Opaque<'a, 'b> {}

// `Opaque<'a, 'b> := Opaque<'static, 'static>`
fn get_iter<'a, 'b>() -> impl IntoIterator<Item = Opaque<'a, 'b>> {
    Some(get_one())
}

Normally when we encounter an equality Opaque<A> == Opaque<A>, we equate substs and never register a hidden type.

The only way we infer an opaque type to be recursive in borrowck is through replace_opaque_types_with_inference_vars here:

rust/compiler/rustc_trait_selection/src/traits/project.rs

Lines 319 to 328 in 82c5732

	// For an example where this is necessary see tests/ui/impl-trait/nested-return-type2.rs
	// This allows users to omit re-mentioning all bounds on an associated type and just use an
	// `impl Trait` for the assoc type to add more bounds.
	let InferOk { value: actual, obligations: new } =
	selcx.infcx.replace_opaque_types_with_inference_vars(
	actual,
	obligation.cause.body_id,
	obligation.cause.span,
	obligation.param_env,
	);

[aliemjay]

A related ICE. TODO report in a separate issue:

#![feature(type_alias_impl_trait)]

trait IsU8 {}
impl IsU8 for u8 {}

struct IterOfU8<T>(T);

trait Iter {
    type Item;
}
impl<T, I> Iter for IterOfU8<T>
where
    T: IntoIterator<Item = I>,
    I: IsU8,
{
    type Item = I;
}

type Opaque<A> = impl Sized;
fn iter_of_opaque<A, T: Iter<Item = Opaque<A>>>() {}
fn test<A>() -> Opaque<A> {
    iter_of_opaque::<A, IterOfU8<Vec<u8>>>();
    0u8
}

[oli-obk]

The first example compiles on master