Trait objects do not work with generic associated types #81823
Description
I tried this code:
#![feature(generic_associated_types)]
trait StreamingIterator {
type Item<'a> where Self: 'a;
fn size_hint(&self) -> (usize, Option<usize>);
// Uncommenting makes `StreamingIterator` not object safe
// fn next(&mut self) -> Self::Item<'_>;
}
fn min_size(x: &mut dyn for<'a> StreamingIterator<Item<'a> = &'a i32>) -> usize {
x.size_hint().0
}I expected to see this happen: either code compiles or an error message is emitted for any use of a trait object with generic associated types
Instead, this happened: code produces inconsistent error messages:
error[E0038]: the trait `StreamingIterator` cannot be made into an object
--> <source>:18:16
|
18 | fn min_size(x: &mut dyn for<'a> StreamingIterator<Item<'a> = &'a i32>) -> usize {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `StreamingIterator` cannot be made into an object
|
= help: consider moving `next` to another trait
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> <source>:15:26
|
11 | trait StreamingIterator {
| ----------------- this trait cannot be made into an object...
...
15 | fn next(&mut self) -> Self::Item<'_>;
| ^^^^^^^^^^^^^^ ...because method `next` references the `Self` type in its return type
error: aborting due to previous error
error: generic associated types in trait objects are not supported yet
--> <source>:19:7
|
19 | x.size_hint().0
| ^^^^^^^^^
error: aborting due to previous error
Compiler returned: 1