Insufficient normalization

[alexreg]

The following should work, as far as I know, but unfortunately doesn't. It seems like the type bound on Bar is getting ignored when used in Foo.

use ::std::io::Write;

pub trait Foo {
    type FooBaz: Bar;
    
    fn get_bars() -> Box<Iterator<Item = Self::FooBaz>>;
}

pub trait Bar where <Self::BarBaz as ToOwned>::Owned: Write {
    type BarBaz: ToOwned;
}

Of course, ideally I would specify the where clause on the associated type itself, but this currently isn't possible.

[steveklabnik]

Triage: this doesn't compile, here's the error:

warning: trait objects without an explicit `dyn` are deprecated
 --> src/lib.rs:6:26
  |
6 |     fn get_bars() -> Box<Iterator<Item = Self::FooBaz>>;
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Iterator<Item = Self::FooBaz>`
  |
  = note: `#[warn(bare_trait_objects)]` on by default

error[E0277]: the trait bound `<<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned: std::io::Write` is not satisfied
 --> src/lib.rs:3:1
  |
3 |   pub trait Foo {
  |   ^            - help: consider further restricting the associated type: `where <<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned: std::io::Write`
  |  _|
  | |
4 | |     type FooBaz: Bar;
5 | |     
6 | |     fn get_bars() -> Box<Iterator<Item = Self::FooBaz>>;
7 | | }
  | |_^ the trait `std::io::Write` is not implemented for `<<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned`
8 | 
9 |   pub trait Bar where <Self::BarBaz as ToOwned>::Owned: Write {
  |   ----------------------------------------------------------- required by `Bar`

error[E0277]: the trait bound `<<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned: std::io::Write` is not satisfied
 --> src/lib.rs:6:5
  |
6 |     fn get_bars() -> Box<Iterator<Item = Self::FooBaz>>;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
  |     |                                                  |
  |     |                                                  help: consider further restricting the associated type: `where <<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned: std::io::Write`
  |     the trait `std::io::Write` is not implemented for `<<<Self as Foo>::FooBaz as Bar>::BarBaz as std::borrow::ToOwned>::Owned`
...
9 | pub trait Bar where <Self::BarBaz as ToOwned>::Owned: Write {
  | ----------------------------------------------------------- required by `Bar`

[alexreg]

@steveklabnik Thanks. Can we tag wg-traits maybe? This really needs a priority now, and ideally fixing soon in any case, given how long it's been open.

[comex]

This also applies when the associated type mentioned by the trait's where clause belongs to a different trait. In that case, a where clause on the associated type would not be a substitute.

Example (playground link):

trait A { type Q; }
trait B where Self: A, <Self as A>::Q: Sync {}
fn foo<T: B>() {} // error