Trait bounds don't work on default associated types

[pythongirl325]

Trait bounds fail for default associated types, but not for re-assigned associated types.

The following code does not compile as seen in this playground.

#![feature(associated_type_defaults)]

trait T {
    type Item = ();
}

struct Foo;
impl T for Foo {}

fn f<I: T>() -> I::Item where I::Item: Default {
    Default::default()
}

fn main(){
    f::<Foo>();
}

This gives the error error[E0277]: the trait bound `<Foo as T>::Item: std::default::Default` is not satisfied

But the code does compile if impl T for Foo {} is replaced with

impl T for Foo {
    type Item = ();
}

as seen on this playground

Meta

rustc --version --verbose:

rustc 1.41.0-nightly (59947fcae 2019-12-08)
binary: rustc
commit-hash: 59947fcae6a40df12e33af8c8c7291014b7603e0
commit-date: 2019-12-08
host: x86_64-unknown-linux-gnu
release: 1.41.0-nightly
LLVM version: 9.0

[Centril]

cc @jonas-schievink

[jonas-schievink]

This will be fixed by #61812, but I'm not adding a test for this issue specifically since there are already plenty (and the normalization of the assoc. type is controlled by flipping a single bool in librustc).