Type mismatch when generics involved, works with new solver

[dodomorandi]

I tried this code:

use std::convert::{Infallible, TryInto};

struct A;

#[derive(Debug)]
struct E;

impl From<Infallible> for E {
    fn from(_: Infallible) -> E {
        unreachable!()
    }
}

fn not_working<T>(t: T) -> Result<(), E>
where
    T: TryInto<A>,
    E: From<T::Error>,
{
    let a: A = t.try_into()?;
    // Uncomment the turbofish to make it compile
    breaking /* ::<A> */(a)
}

fn breaking<T>(t: T) -> Result<(), E>
where
    T: TryInto<A>,
    E: From<T::Error>,
{
    let a = t.try_into()?;
    Ok(())
}

I expected to see this happen: the code should compile, the a inside not_working has a type of A and the invocation of breaking is expected to use T = A.

Instead, this happened: the code does not compile with the following error:

error[E0271]: type mismatch resolving `<A as TryInto<A>>::Error == <T as TryInto<A>>::Error`
  --> src/lib.rs:21:5
   |
21 |     breaking /* ::<A> */(a)
   |     ^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found `Infallible`
   |
   = note: expected associated type `<T as TryInto<A>>::Error`
                         found enum `Infallible`
help: consider constraining the associated type `<T as TryInto<A>>::Error` to `Infallible`
   |
16 |     T: TryInto<A, Error = Infallible>,
   |                 ++++++++++++++++++++

If the function which invokes breaking creates the instance of A without using generics, the code compiles:

fn working(a: A) -> Result<(), E> {
    breaking(a)
}

Some interesting things:

• if breaking is invoked specifying the generic with the turbofish notation, the program compiles
• the code compiles in nightly if the -Z next-solver is used
• it is broken at least since 1.34 (I tested this version because of the introduction of TryInto
• it is broken for all editions (but it should compile for all of them)

I am not sure if it could be related to #139531.

Meta

rustc --version --verbose:

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

It is also broken on nightly:

cargo 1.89.0-nightly (68db37499 2025-05-22)
release: 1.89.0-nightly
commit-hash: 68db37499f2de8acef704c73d9031be6fbcbaee4
commit-date: 2025-05-22
host: x86_64-unknown-linux-gnu
libgit2: 1.9.0 (sys:0.20.2 vendored)
libcurl: 8.12.1-DEV (sys:0.4.80+curl-8.12.1 vendored ssl:OpenSSL/3.5.0)
ssl: OpenSSL 3.5.0 8 Apr 2025
os: Arch Linux Rolling Release [64-bit