Missing a warning about overly broad return type declarations

### Description

I expected a warning in this case, but didn't get one:
```
function test(): string|false
{
    return 'some';
}
```

### Playground Link

https://mago.carthage.software/playground#019d5d00-a37a-436c-1904-c12486e59f5b

### Reproduction

_No response_

### Environment

mago current as of this morning.