Strange type inference for recursive RPIT

I tried this code:

#![allow(unconditional_recursion)]

fn what1<T>(x: T) -> impl Sized {
    what1(x)
}

fn what2<T>(x: T) -> impl Sized {
    what2(what2(x))
}

fn print_return_type<T, U>(_: impl Fn(T) -> U) {
    println!("{}", std::any::type_name::<U>())
}

fn main() {
    print_return_type(what1::<i32>);
    print_return_type(what2::<i32>);
}

I compiled the above code in edition 2024, and got the output:

()
i32

Also, the following code gives a compile error:

#![allow(unconditional_recursion)]

fn what3<T>(x: T) -> impl Sized {
    what3(what3(what3(x)))
}

error[E0792]: expected generic type parameter, found `impl Sized`
 --> src/lib.rs:4:5
  |
3 | fn what3<T>(x: T) -> impl Sized {
  |          - this generic parameter must be used with a generic type parameter
4 |     what3(what3(what3(x)))
  |     ^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0792`.

(See also #139350)

I think that both what1 and what2 should not compile, but I'm not sure. And if it does compiles, they should both return !. However, they return () and i32, respectively. (This happens despite the edition 2024 never type fallback change.) The current behavior doesn't make any sense to me.

Discovered while experimenting with #139402.

@rustbot labels +A-impl-trait

Meta

Reproducible on the playground with nightly rust 1.88.0-nightly (2025-04-04 17ffbc81a30c09419383)

Start with the src/lib.rs reproducer on the referenced nightly compiler and compare the inferred return types of what1, what2, and what3, including the E0792 diagnostic. Read the discussion in #139350 and the related context from #139402, then determine the intended recursive RPIT and never-type-fallback behavior. Done means the behavior is resolved or clearly specified with regression coverage.