# XO0202: error type does not convert to the enclosing error type

`expr?` on a `Result[T, E2]` returns the error from the enclosing function, so `E2` must convert to its error type `E`: `E2` is `E`, or `E` is `Error` and `E2` implements `Display`, or `E` has a `from E2` variant. Add a `from` variant, derive `Display` for an `Error` target, or convert inline with `map_err`. The same rule applies inside a closure whose error type is fixed. See core 2.6.

## Example

```xo
enum LoadErr { Timeout }
enum ApiErr { Bad }

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

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

## Fix

```xo
enum LoadErr { Timeout }
enum ApiErr { Bad, Load(from LoadErr) }

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

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

Run `xo explain XO0202` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).