Possible regression: Updating 2.8.0 to 2.9.0 causes "type annotations needed" for BTreeSet::range etc.

[TonalidadeHidrica]

See the following code:

use std::collections::BTreeSet;
use ordered_float::NotNan;

fn main() {
    let a = BTreeSet::<NotNan<f64>>::new();
    let b = a.range(..);
}

In 2.8.0, this code compiles. In 2.9.0 and 2.10.0, it causes the following error:

Copyable text

error[E0283]: type annotations needed
 --> src/main.rs:6:15
  |
6 |     let b = a.range(..);
  |               ^^^^^ cannot infer type for type parameter `K` declared on the associated function `range`
  |
  = note: cannot satisfy `_: Ord`

I cannot figure out at all why this error happens, but this is indeed a regression.

$ rustc --version
rustc 1.57.0 (f1edd0429 2021-11-29)

[TonalidadeHidrica]

The workaround is to explicitly specify the type parameter K.

use std::collections::BTreeSet;
use ordered_float::NotNan;

fn main() {
    let a = BTreeSet::<NotNan<f64>>::new();
    let b = a.range::<NotNan<f64>, _>(..);
}

[mbrubeck]

Hmm, this was caused by #98.

[TonalidadeHidrica]

Isn't it strange that implementing a trait make type inference fail? I'm wondering if this tends to happen, or it is a bug (or something can be improved) of Rust compiler.

[mbrubeck]

The BTreeSet::range method is generic over a type K: Ord where T: Borrow<K> + Ord. So it seems plausible that adding a new Borrow implementation to NotNan could allow calling this function with additional types.

However, NotNan<f64> only implements Borrow<f64>, and f64 does not implement Ord. So it should not affect this function call. It seems the compiler is not smart enough to see this, or it is overly conservative since it believes that f64 might implement Ord in the future. I'm not sure which.

[mbrubeck]

Closed accidentally. I meant to close #91, not this one.