failure to normalize associated types in supertraits

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)) { }

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.

The example compiles with 1.73.

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

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

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Rollup merge of rust-lang#160338 - zakrad:regr-test-61083, r=JohnTitor

Unrolled build for #160338

Rollup merge of #160338 - zakrad:regr-test-61083, r=JohnTitor

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failure to normalize associated types in supertraits #61083

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	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();

failure to normalize associated types in supertraits #61083

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nikomatsakis commented on May 23, 2019

istankovic commented on Oct 20, 2023

fmease commented on Sep 18, 2025

lukaslueg commented on Oct 2, 2025

zakrad commented on Jul 30, 2026

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