```python
def is_gap(reading: int) -> bool:
    return reading == -1


def fill_gaps(readings: list) -> list:
    """Missing readings replaced by the last real one seen."""
    filled = list(readings)
    for i in range(len(filled)):
        if is_gap(filled[i]):
            if i == 0:
                filled[i] = -1  # Missing reading before any real one stays missing
            else:
                filled[i] = filled[i - 1]
    return filled
```