# E4158

Default implementation is required but not found.

When a trait declares that it has a default implementation, the implementation
should exist.

## Erroneous example

```{literalinclude} /sources/error_codes/4158_error/top.mbt
:language: moonbit
```

## Suggestion

Implement the default implementation.

```{literalinclude} /sources/error_codes/4158_fixed/example.mbt
:language: moonbit
```