"conflicting implementations of trait" for two distinct types

[Dzuchun]

I tried this code:

trait T {}
impl T for <&'static i32 as core::ops::Deref>::Target {}
impl T for () {}

I expected to see this happen: successful compilation, since <&'static i32 as core::ops::Deref>::Target is i32 (clearly not ())

Instead, this happened: compilation failed with

error[E0119]: conflicting implementations of trait `T` for type `<&'static i32 as Deref>::Target`
 --> src/lib.rs:3:1
  |
2 | impl T for <&'static i32 as core::ops::Deref>::Target {}
  | --------------------------------------------- first implementation here
3 | impl T for () {}
  | ^^^^^^^^^^^^^ conflicting implementation for `<&'static i32 as Deref>::Target`

It should be noted, that substituting <&'static i32 as Deref>::Target for i32 removes the error, i.e. the following code compiles:

trait T {}
impl T for i32 {}
impl T for () {}

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

In my testing, all of the following reproduced this behavior:

• stable 1.87.0
• beta 1.88.0-beta.5 (2025-06-01)
• nightly 1.89.0-nightly (2025-06-11)

(no backtrace produced)

[Dzuchun]

After further testing, I realized that this might be the intended behavior to prevent code breaking on foreign associated type change. Following code compiles:

trait V {
    type U;
}
impl V for () {
    type U = i32;
}

trait T {}
impl T for <() as V>::U {}
impl T for () {}

I haven't seen this pointed out anywhere though.

[Dzuchun]

It appears there exists a similar issue (#51445), documenting this exact behavior.