Rust treat Self as dynamic when it shouldn't

[Pzixel]

Here is a sample code:

trait Inner<S> {}

trait MyTrait {
    fn something<I: Inner<Self>>(i: I);
}

Which fails to compile with

error[E0277]: the size for values of type Self cannot be known at compilation time
--> src/lib.rs:4:5
|
4 | fn something<I: Inner>(i: I);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

OTOH if we add a special This associated type then we are completely fine:

#![feature(associated_type_defaults)]

trait Inner<S> {}

trait MyTrait {
    type This = Self;
    fn something<I: Inner<Self::This>>(i: I);
}

It looks like an undesirable behavior.

[jonas-schievink]

That this fails to compile is intended. Unlike type parameters, traits do not have a default : Sized bound, so Self in traits has an unknown size.

The issue with associated type defaults might be fixed by #61812, I'll check that later.