All relaxed bounds are assumed to be ?Sized

[ibraheemdev]

fn foo() -> impl Iterator<Item = u8> + ?Send {
    "".to_string().into_bytes().into_iter()
}

fn main() {
    let x = foo();
}

// This warning is correct
warning: default bound relaxed for a type parameter, but this does nothing because the given bound is not a default; only `?Sized` is supported
 --> src/main.rs:1:13
  |
1 | fn foo() -> impl Iterator<Item = u8> + ?Send {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

// These errors aren't
error[E0277]: the size for values of type `impl Iterator+?Sized` cannot be known at compilation time
 --> src/main.rs:1:13
  |
1 | fn foo() -> impl Iterator<Item = u8> + ?Send {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `impl Iterator+?Sized`
  = note: the return type of a function must have a statically known size

error[E0277]: the size for values of type `impl Iterator+?Sized` cannot be known at compilation time
 --> src/main.rs:6:9
  |
6 |     let x = foo();
  |         ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `impl Iterator+?Sized`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

error[E0277]: the size for values of type `impl Iterator+?Sized` cannot be known at compilation time
 --> src/main.rs:6:13
  |
6 |     let x = foo();
  |             ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `impl Iterator+?Sized`
  = note: the return type of a function must have a statically known size

error: aborting due to 3 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

To learn more, run the command again with --verbose.