Make 'datetime' a subclass of 'date'. - #2488

The incompatibilities I've found have to do with the date's comparison dunders.

I'd like to include some tests; but I don't find the relevant files in mypy. I've manually tested with:

from datetime import date, datetime, timedelta

class Date: ...
class Datetime(Date): ...

now: date = datetime.now()
rightnow: Date = Datetime()

# Check __sub__
now3: datetime = datetime.now()
maybe_zero: timedelta = now3 - datetime.now()
now2: datetime = now3 + maybe_zero

now4: date = now3 + maybe_zero
now5: datetime = now4 + maybe_zero  # Incompatible types in assignment

# Check __lt__ and friends
now4 < now5, now4 <= now5, now4 > now5, now4 >= now5
now5 < now4, now5 <= now4, now5 > now4, now5 >= now4

With mypy 0.630 I get:

bug.py:6: error: Incompatible types in assignment (expression has type "datetime", variable has type "date")
bug.py:16: error: Incompatible types in assignment (expression has type "datetime", variable has type "date")
bug.py:17: error: Incompatible types in assignment (expression has type "date", variable has type "datetime")
bug.py:20: error: Unsupported operand types for < ("date" and "datetime")
bug.py:20: error: Unsupported operand types for <= ("date" and "datetime")
bug.py:20: error: Unsupported operand types for > ("date" and "datetime")
bug.py:20: error: Unsupported operand types for >= ("date" and "datetime")
bug.py:21: error: Unsupported operand types for < ("datetime" and "date")
bug.py:21: error: Unsupported operand types for <= ("datetime" and "date")
bug.py:21: error: Unsupported operand types for > ("datetime" and "date")
bug.py:21: error: Unsupported operand types for >= ("datetime" and "date")

With this PR I only get the error in line 17.

I'd like to note that I was surprised when I updated typeshed and mypy stopped enforcing the distinction between Date and DateTime (specifically, I have functions meant to operate on Date where DateTime arguments should be rejected).

They do indeed have a subtyping relationship in Python, but I can't think of a time when I wanted to be able to pass a DateTime where only Date was expected. The two classes do seem to be largely compatible (barring differences in result of .isoformat() and edge cases with, e.g., adding 24 hours across a timezone boundary and then normalizing).

I'm not really sure whether or not it should be changed back, but it seems worth at least having the discussion?

Since that relationship exists in Python, I believe it should be represented in typeshed. Otherwise this can lead to false positives for people who rely on this behaviour.