Trait associated types `where Self: Sized` interact oddly with dyn safety

[CAD97]

Given the trait

pub trait Trait {
    type Assoc
    where
        Self: Sized;

    fn method(&self);
}

we get that

• &dyn Trait is not valid,

  error[E0191]: the value of the associated type `Assoc` (from trait `Trait`) must be specified
   --> src/lib.rs:9:21
    |
  2 |     type Assoc
    |     ---------- `Assoc` defined here
  ...
  9 | pub fn test(t: &dyn Trait) {
    |                     ^^^^^ help: specify the associated type: `Trait<Assoc = Type>`

• &dyn Trait<Assoc=()> is valid,
• but trying to call methods fails.

  error[E0277]: the size for values of type `dyn Trait<Assoc = ()>` cannot be known at compilation time
    --> src/lib.rs:10:5
     |
  10 |     t.method();
     |     ^ ------ required by a bound introduced by this call
     |     |
     |     doesn't have a size known at compile-time
     |
     = help: the trait `Sized` is not implemented for `dyn Trait<Assoc = ()>`

The error message says that the call to Trait::method introduces a Self: Sized bound. There is no such bound on method, however; the bound is on Trait::Assoc.

I have no clue what the "correct" behavior is for this, but it's probably not this.

[fmease]

The code you posted:

pub trait Trait {
    type Assoc
    where
        Self: Sized;

    fn method(&self);
}

nowadays compiles successfully, namely since oli's PR #112319. Closing as completed then.