Inherent associated types are incorrectly substituted

[fmease]

The following code should compile since for Subj<(i32,)> one has T == i32 and thus Subj::Un == i32. However, the current implementation incorrectly substitutes T with (i32,) leading to a compile error after encountering the let-statement.

#![feature(inherent_associated_types)]
#![allow(incomplete_features)]

struct Subj<T>(T);

impl<T> Subj<(T,)> {
    type Un = T;
}

fn main() {
    let _: Subj::<(i32,)>::Un = 0i32;
}

error[E0308]: mismatched types
  --> src/main.rs:11:33
   |
11 |     let _: Subj::<(i32,)>::Un = 0i32;
   |            ------------------   ^^^^ expected tuple, found `i32`
   |            |
   |            expected due to this
   |
   = note: expected tuple `(i32,)`
               found type `i32`
help: use a trailing comma to create a tuple with one element
   |
11 |     let _: Subj::<(i32,)>::Un = (0i32,);
   |                                 +    ++

---

For comparison, the analogous program involving inherent associated functions successfully compiles:

struct Subj<T>(T);

impl<T: Default> Subj<(T,)> {
    fn un() -> T { T::default() }
}

fn main() {
    let _: i32 = Subj::<(i32,)>::un();
}

@rustbot label T-compiler requires-nightly F-inherent_associated_types
@rustbot claim

[compiler-errors]

same root cause, please add this as a test when you fix this @fmease:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b267739cce2e6be9f50bb4bf0c3dbd85

[fmease]

#105305 related