Cow trait bound without ToOwned produces odd "core::marker::Sized is not implemented"

[comex]

Incorrect code, because Cow requires ToOwned on its parameter:

fn into_cow<'a, T: ?Sized, S: Into<Cow<'a, T>>>(s: S) -> Cow<'a, T> {
    s.into()
}

Fixed:

fn into_cow<'a, T: ?Sized + ToOwned, S: Into<Cow<'a, T>>>(s: S) -> Cow<'a, T>
                          ^^^^^^^^^

Makes sense, but rustc's error for the first version is odd:

exectool.rs:46:1: 48:2 error: the trait `core::marker::Sized` is not implemented for the type `T` [E0277]
exectool.rs:46 fn into_cow<'a, T: ?Sized, S: Into<Cow<'a, T>>>(s: S) -> Cow<'a, T> {
exectool.rs:47     s.into()
exectool.rs:48 }
exectool.rs:46:1: 48:2 help: run `rustc --explain E0277` to see a detailed explanation
exectool.rs:46:1: 48:2 note: `T` does not have a constant size known at compile-time
exectool.rs:46:1: 48:2 note: required by `core::convert::Into`

Into doesn't require T to be Sized, and indeed in the Cow case it usually isn't.

If I remove the ?Sized from T, I get the trait core::clone::Clone is not implemented for the type T; stable produces this error for the original version too, along with the Sized error. On IRC, aatch suggested this was might be caused by impl<T: Clone> ToOwned for T. While Clone would be a sufficient bound due to that impl, it isn't necessary even for sized types, so that error too is misleading.

[Aatch]

Related: #30976

[Mark-Simulacrum]

Seems like a duplicate of #64088 essentially