Match arms don't work with ()? in one arm and Ok(()) in the other

[ghost]

My rustc version: 1.25.0.

---

Currently this doesn't compile:

use std::fmt::Display;
use std::fmt::Error;
use std::fmt::Formatter;

struct A { s: Option<String>, }

impl Display for A {

    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        match self.s {
            // This is either Ok(()) or Err(_)
            Some(ref s) => write!(f, "{}", s)?,
            // This is obviously Ok(())
            None => Ok(()),
        }
    }

}

Error message:

error[E0308]: match arms have incompatible types
  --> tests/src/main.rs:13:28
   |
13 |             Some(ref s) => write!(f, "{}", s)?,
   |                            ^^^^^^^^^^^^^^^^^^^
   |                            |
   |                            expected enum `std::result::Result`, found ()
   |                            match arm with an incompatible type
   |
   = note: expected type `std::result::Result<(), std::fmt::Error>`
              found type `()`

error: aborting due to previous error

error: Could not compile `tests`.

Work around:

impl Display for A {

    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        match self.s {
            Some(ref s) => write!(f, "{}", s)?,
            None => (),
        };
        Ok(())
    }

}

I think the work around looks... weird, where one arm returns Result<>, the other returns (). While the first code looks pretty clear: both arms return Result<>.

---

Perhaps #24157 is related, I'm not sure.

Thank you,

[jonas-schievink]

write!(f, "{}", s)? doesn't evaluate to a Result, it evaluates to (), so the error is correct.

The ? operator creates code that's roughly equivalent to this:

match write!(f, "{}", s) {
    Ok(val) => val,
    Err(e) => return e.into(),
}

[ghost]

@jonas-schievink Thank you very much :-) I'm closing this issue.