Preserve the sign of NaNs when converted to f32?

[shaobo-he]

Hello,

I noticed that the sign of f32 NaNs are preserved when they are converted to f64 as indicated by this experiment: https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=b23459b96256c673aa6d84ed79a10f50.

But it appears f16 NaNs do not have this behavior. For example, the second assertion fails.

extern crate half;

use half::*;

fn main() {
    let x = f16::from_bits(64513);
    let y = x.to_f32();
    assert!(x.is_sign_negative());
    println!("{}", y);
    assert!(y.is_sign_negative());
}

I couldn't find any useful information about whether the standard specifies this. So I'm not sure if f16 should be consistent with f32 with respect to this behavior.