# XO0205: `?` where the enclosing function cannot return the failure

`?` on a `Result` needs a function or closure that can fail, and `?` on an optional needs one that returns an optional. A function without `!` cannot propagate an error, and a `requires ... else` clause in such a function has nowhere to return its error either. Declare the error type (the diagnostic offers `! E` as a fix), or handle the failure with `match`, `??`, or `ignore_err`. See core 2.5 and 2.6.

## Example

```xo
enum E { A }

fn g() -> Int ! E { 1 }

fn f() -> Int { g()? }
```

## Fix

```xo
enum E { A }

fn g() -> Int ! E { 1 }

fn f() -> Int ! E { g()? }
```

Run `xo explain XO0205` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).