Confusing behavior and mismatched error reports for `impl_trait_in_fn_trait_return` && `type_alias_impl_trait`

I tried this code in my lib for curring function with generics:

#![feature(impl_trait_in_fn_trait_return)]
#![feature(type_alias_impl_trait)]
#![allow(clippy::let_and_return)]

use std::fmt::Display;

// Failed to compile
fn concat_1<T: Display>(a: T) -> impl FnOnce(T) -> impl FnOnce(T) -> String {
    move |b: T| move |c: T| format!("{a} {b} {c}")
}

// Successful
fn concat_2<T: Display>(a: T) -> impl FnOnce(T) -> impl FnOnce(T) -> String {
    let f = move |b: T| move |c: T| format!("{a} {b} {c}");
    f
}

// Failed to compile
type A3<T: Display> = impl FnOnce(T) -> B3<T>;
type B3<T: Display> = impl FnOnce(T) -> String;
fn concat_3<T: Display>(a: T) -> A3<T> {
    move |b: T| move |c: T| format!("{a} {b} {c}")
}

// Successful
type A4<T: Display> = impl FnOnce(T) -> B4<T>;
type B4<T: Display> = impl FnOnce(T) -> String;
fn concat_4<T: Display>(a: T) -> A4<T> {
    let f = move |b: T| move |c: T| format!("{a} {b} {c}");
    f
}

fn main() {}

Meta

rustc --version --verbose:

rustc 1.78.0-nightly (2d24fe591 2024-03-09)
binary: rustc
commit-hash: 2d24fe591f30386d6d5fc2bb941c78d7266bf10f
commit-date: 2024-03-09
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

Error output

• for concat_1:

error: concrete type differs from previous defining opaque type use
 --> src/main.rs:9:17
  |
9 |     move |b: T| move |c: T| format!("{a} {b} {c}")
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `impl FnOnce(T) -> String`, got `{closure@src/main.rs:9:17: 9:28}`
  |
note: previous use here
 --> src/main.rs:9:5
  |
9 |     move |b: T| move |c: T| format!("{a} {b} {c}")
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0720]: cannot resolve opaque type
 --> src/main.rs:8:52
  |
8 | fn concat_1<T: Display>(a: T) -> impl FnOnce(T) -> impl FnOnce(T) -> String {
  |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^ cannot resolve opaque type

For more information about this error, try `rustc --explain E0720`.

• for concat_3:

error: concrete type differs from previous defining opaque type use
  --> src/main.rs:22:17
   |
22 |     move |b: T| move |c: T| format!("{a} {b} {c}")
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `B3<T>`, got `{closure@src/main.rs:22:17: 22:28}`
   |
note: previous use here
  --> src/main.rs:22:5
   |
22 |     move |b: T| move |c: T| format!("{a} {b} {c}")
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Why there is E0720 error in a piece of code where recursion does not occur?
(An impl Trait type must be expandable to a concrete type that contains no impl Trait types.)

Temporary solution:

fn concat<T: Display + 'static>(a: T) -> Box<dyn FnOnce(T) -> Box<dyn FnOnce(T) -> String>> {
    Box::new(move |b: T| Box::new(move |c: T| format!("{a} {b} {c}")))
}

playground

Reproduce the four examples in src/main.rs with the nightly compiler version reported in the issue, and compare the diagnostics for concat_1 through concat_4. Use the E0720 output and the linked playground to trace the mismatch between nested opaque types and closures; done means the behavior is explained or the compiler reports the cases consistently.