Arithmetic operations with None not detected without strict-optional

[pkch]

(Based on #3111)

Without --strict-optional, some invalid None opertations are undetected:

None + 1 # ok
None * 1 # ok
None / 1 # ok
None - 1 # ok

None + '' # error
None * '' # ok
None / '' # error
None - '' # error

With --strict-optional, all of them fail type check but with inconsistent error messages:

Unsupported operand types for + (None and "int")
Unsupported operand types for * (None and "int")
Unsupported operand types for / (None and "int")
Unsupported operand types for - (None and "int")
Unsupported left operand type for + (None)
Unsupported operand types for * (None and "str")
Unsupported left operand type for / (None)
Unsupported left operand type for - (None)

[ilevkivskyi]

The plan is to make --strict-optional turned on by default, so that the first part does not need to be fixed. However, the error messages could be more homogeneous indeed.

[pkch]

Then I guess we should fix a few tests because everyone likes to put None + 1 as an example of what should fail (like in testTypeCheckNamedModule2, testMypyConditional, etc.). Or are we going to start running tests with --strict-optional in the next few days?

[ilevkivskyi]

Or are we going to start running tests with --strict-optional in the next few days?

No, this is rather a long-term goal. However, there are dedicated tests for this flag: check-optional.test

[msullivan]

Strict optional is the default now, so I'm going to say this is "working as intended"