impl Trait in return type does not compile if function returns panic!() / todo!() / unimplemented!()

[UditDey]

The following code fails to compile:

trait Trait {}

struct Struct;

impl Trait for Struct {}

fn example() -> impl Trait {
    panic!()
}

error[E0277]: the trait bound `(): Trait` is not satisfied
 --> src/main.rs:7:17
  |
7 | fn example() -> impl Trait {
  |                 ^^^^^^^^^^ the trait `Trait` is not implemented for `()`
  |
  = help: the trait `Trait` is implemented for `Struct`

However, replacing the impl Trait with the regular generics syntax compiles fine:

fn example<T: Trait>() -> T {
    panic!()
}

This happens only if there is a single path in the function terminating in a panic!(). The following compiles fine too:

fn example(b: bool) -> impl Trait {
    if b {
        Struct {}
    }
    else {
        panic!()
    }
}

This also happens with todo!() and unimplemented!() since those macros are also basically just panic!()s.

If my understanding is correct, impl Trait and <T: Trait> should behave identically, but clearly they don't and the typecheck fails in the impl Trait version. This means that, for example, using todo!() in place of unfinished code is not possible in a function returning impl Trait.

This occurs on 1.64.0 stable as well as nightly

[UditDey]

The same behavior can be seen with !. With impl Trait this does not compile, but replacing it with <T: Trait> works fine:

trait Trait {}

struct Struct;

impl Trait for Struct {}

fn never_return() -> ! {
    loop {}
}

fn example() -> impl Trait {
    never_return()
}

In fact, the error message seems to be incorrect as well:

error[E0277]: the trait bound `(): Trait` is not satisfied
  --> src/main.rs:11:17
   |
11 | fn example() -> impl Trait {
   |                 ^^^^^^^^^^ the trait `Trait` is not implemented for `()`
   |
   = help: the trait `Trait` is implemented for `Struct`

Shouldn't it be trait bound '!: Trait' is not satisfied? If the return type of panic!() is taken as ! then this is where the problem stems from

[memoryruins]

Duplicate of #36375

[thomcc]

Closed as a duplicate.