fn foo<T:?>() {} compiles despite the weird trait bound

[scottmcm]

Both of the following compile fine https://is.gd/k1iXTl:

fn foo<T:?>() {}

use std::error::Error;
fn bar<E>() -> Result<(),E>
    where E : Error + ?
{
    Ok(())
}

I would have expected that a lonely ? (without a trait name) in a bound was an error.

(Thanks @tomaka on IRC for the simpler repro.)

[est31]

Interestingly, this reproduces on stable, but not on beta or nightly. Here both is an error.

[kennytm]

Fixed in #41077.

https://github.com/rust-lang/rust/blob/master/src/test/parse-fail/bound-single-question-mark.rs