`const_fn_trait_bound` causes rustc not to recognize trait bounds

[js2xxx]

I replaced lib.rs with this code after creating an empty lib:

#![no_std]
#![feature(const_btree_new)]
#![feature(const_fn_trait_bound)]

use alloc::collections::BTreeMap;

extern crate alloc;

pub struct CustomMap<K, V>(BTreeMap<K, V>);

impl<K, V> CustomMap<K, V>
where
    K: Ord,
{
    pub const fn new() -> Self {
        CustomMap(BTreeMap::new())
    }
}

When running cargo build, I expected to see this happen: built successfully

Instead, this happened:

   Compiling failure-test v0.1.0 (/home/js2xxx/Projects/failure-test)
error[E0277]: the trait bound `K: Ord` is not satisfied
   --> src/lib.rs:16:19
    |
16  |         CustomMap(BTreeMap::new())
    |                   ^^^^^^^^^^^^^^^
    |                   |
    |                   expected an implementor of trait `Ord`
    |                   help: consider borrowing here: `&BTreeMap::new()`
    |
note: required by `BTreeMap::<K, V>::new`
   --> /home/js2xxx/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/collections/btree/map.rs:512:5
    |
512 | /     pub const fn new() -> BTreeMap<K, V>
513 | |     where
514 | |         K: Ord,
    | |_______________^

For more information about this error, try `rustc --explain E0277`.
error: could not compile `failure-test` due to previous error

And when I followed the suggestions it offered, it turned into a type mismatch error and wanted me to cancel the borrowing.

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (8007b506a 2021-08-14)
binary: rustc
commit-hash: 8007b506ac5da629f223b755f5a5391edd5f6d01
commit-date: 2021-08-14
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1

The output of RUST_BACKTRACE=1 cargo build is the same as above.

[jyn514]

So this is odd: if you move the Ord bound to the function it works fine: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=1bec0846e3c540afc36b436b508947f1

and if you add both bounds it stops working, which is pretty incomprehensible to me: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=fcd4b29d4f01dba3eacaaacb0cdbae5e

[jyn514]

This is definitely related to const_fn_trait_bound, because removing const from the function makes it work fine.

[Kazurin-775]

It seems that you don't even need to specify a trait bound in your code to cause this bug. The following code breaks for me:

pub struct PidPool {
    pool: BTreeSet<u32>,
}

impl PidPool {
    pub const fn new() -> PidPool {
        PidPool {
            pool: BTreeSet::new(),
        }
    }
}

which gives the following very weird output on nightly:

error[E0277]: the trait bound `u32: Ord` is not satisfied
   --> src/lib.rs:10:19
    |
10  |             pool: BTreeSet::new(),
    |                   ^^^^^^^^^^^^^ the trait `Ord` is not implemented for `u32`
    |
note: required by `BTreeSet::<T>::new`
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
    |
7   | impl PidPool where u32: Ord {
    |              ++++++++++++++

(But anyway, there is a trait bound specified in BTreeSet's source code. Hope this code helps in diagnosing this problem.)

[Kazurin-775]

Update: I'm not quite sure what happened, but it seems that this problem is already fixed in the latest nightly, and both code snippets can now be compiled without problem. Maybe it's time to close this issue.

(This issue should be some kind of regression introduced at some point between nightly-2021-08-14 and nightly-2021-08-15, according to my tests.)