Not unsize-coercing leads to "the size for values of type dyn Trait cannot be known at compilation time"

[theemathas]

Code

trait Trait {}

struct Thing;
impl Trait for Thing {}

impl<T: Trait, E> Trait for Result<T, E> {}

impl<T: Trait + ?Sized> Trait for Box<T> {}

fn foo<T>(x: &T) -> impl Trait {
    if true {
        Ok(Box::new(foo(x)))
    } else {
        Ok(Box::new(Thing) as Box<dyn Trait>)
    }
}

Current output

error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
  --> src/lib.rs:14:9
   |
14 |         Ok(Box::new(Thing) as Box<dyn Trait>)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn Trait`

For more information about this error, try `rustc --explain E0277`.

Desired output

Rationale and extra context

I have no idea what the compiler is trying to tell me here.

Other cases

trait Trait {}

struct Thing;
impl Trait for Thing {}

impl<T: Trait + ?Sized> Trait for Box<T> {}

fn foo<T>(x: &T) -> impl Trait {
    if true {
        Box::new(foo(x))
    } else {
        Box::new(Thing) as Box<dyn Trait>
    }
}

Output:

error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
  --> src/lib.rs:12:9
   |
12 |         Box::new(Thing) as Box<dyn Trait>
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn Trait`

error: concrete type differs from previous defining opaque type use
  --> src/lib.rs:8:21
   |
8  | fn foo<T>(x: &T) -> impl Trait {
   |                     ^^^^^^^^^^ expected `dyn Trait`, got `Box<impl Trait>`
   |
note: previous use here
  --> src/lib.rs:12:9
   |
12 |         Box::new(Thing) as Box<dyn Trait>
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0277`.

Rust Version

Reproducible on playground with rust 1.87.0-nightly (2025-02-24 617aad8c2e8783f6df8e)

Anything else?

Minimized from code written by starwort on the rust community discord.

Error output using edition 2024.