A wrong type is reported in a type mismatch error

Consider this code:
```fsharp
let a, b: int = ()
```

The error is produced on the `unit` expression, but it says that the type is `int`:

```
Error FS0001 : This expression was expected to have type
''a * 'b'
but here has type
'int'
```

This may be happening due to some expressions rewriting during the analysis.