Code

I tried this code:

pub trait FieldElement { type Integer: TryFrom<usize, Error = std::num::TryFromIntError>; fn valid_integer_try_from<N>(i: N) -> Result<Self::Integer, ()> where Self::Integer: TryFrom<N>, { Self::Integer::try_from(i).map_err(|_| ()) } }

I expected to see this happen: Successful compilation.

Instead, this happened: I now get the following compiler error.

error[E0308]: mismatched types --> src/lib.rs:8:33 | 4 | fn valid_integer_try_from<N>(i: N) -> Result<Self::Integer, ()> | - this type parameter ... 8 | Self::Integer::try_from(i).map_err(|_| ()) | ----------------------- ^ expected `usize`, found type parameter `N` | | | arguments to this function are incorrect | = note: expected type `usize` found type parameter `N` note: associated function defined here --> /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:478:8 | 478 | fn try_from(value: T) -> Result<Self, Self::Error>; | ^^^^^^^^ For more information about this error, try `rustc --explain E0308`. error: could not compile `nightly-prio-regression` due to previous error 

Version it worked on

It most recently worked on: stable channel Rust 1.63.0, and nightly-2022-08-17.

Version with regression

searched nightlies: from nightly-2022-08-11 to nightly-2022-08-26
regressed nightly: nightly-2022-08-18
searched commit range: 86c6ebe...9c20b2a
regressed commit: 9c20b2a

cargo bisect-rustc 

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

While reducing this example, I noticed that the following changes to the code will make the error go away:

• Replacing the function call with N::try_into(i)
• Deleting the specification of Error in the associated type's trait bound
• Adding the same Error specification in the where clause's trait bound