failure to normalize associated types in supertraits

[nikomatsakis]

This example does not compile, but I believe it ought to (playground):

use std::vec::Vec;

trait Foo<T> { }

trait Bar<T: Iterator>: Foo<T::Item> { }

fn a(x: &(impl Foo<u32> + ?Sized)) {
    
}

fn b(y: &dyn Bar<std::vec::IntoIter<u32>>) {
    a(y)
}

fn main() {
    
}

we get the error:

error[E0277]: the trait bound `dyn Bar<std::vec::IntoIter<u32>>: Foo<u32>` is not satisfied
  --> src/main.rs:12:5
   |
12 |     a(y)
   |     ^ the trait `Foo<u32>` is not implemented for `dyn Bar<std::vec::IntoIter<u32>>`
   |
note: required by `a`
  --> src/main.rs:7:1
   |
7  | fn a(x: &(impl Foo<u32> + ?Sized)) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Note that changing b like so compiles (playground):

fn b(y: &(impl Bar<std::vec::IntoIter<u32>> + ?Sized)) { }

[nikomatsakis]

The problem seems to be here:

rust/src/librustc/traits/select.rs

Lines 2163 to 2171 in f688ba6

	let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
	.filter(|upcast_trait_ref| {
	self.infcx.probe(|_| {
	let upcast_trait_ref = upcast_trait_ref.clone();
	self.match_poly_trait_ref(obligation, upcast_trait_ref)
	.is_ok()
	})
	})
	.count();

util::supertraits yields up unnormalized trait references, in particular.

[istankovic]

The example compiles with 1.73.

[fmease]

Might or might not need a regression test. Bisecting to the PR that fixed it might clear that up.

[lukaslueg]

Compiles since nightly-2024-11-27

get_commits_between returning commits, len: 4
commit[0] 2024-11-25: Auto merge of #133247 - GuillaumeGomez:reduce-integer-display-impl, r=workingjubilee
commit[1] 2024-11-26: Auto merge of #133465 - ehuss:update-cargo, r=ehuss
commit[2] 2024-11-26: Auto merge of #132894 - frank-king:feature/where-refactor, r=cjgillot
commit[3] 2024-11-26: Auto merge of #133500 - GuillaumeGomez:rollup-4vcthwo, r=GuillaumeGomez