Use union instead of join for unifying types

[JelleZijlstra]

@erictraut has gathered a long list of problems caused by mypy's behavior of using the "join" operator to unify types in various contexts.

• Incompatible return value type (got "function", expected "Callable[..., Any]") #12053
• Attribute of type A | B | None becomes object after narrowing checks #12009
• enumerate causes incompatible type mypy error #11934
• a joining of two types becomes object not a union #11440
• Unable to remove Optional wrapper when using Mapping[str, str] instead of dict #11618
• Expression has type "function", variable has type "Callable[[], None]" #10740
• Join of types depends on order of types #10442
• Incorrect type reification on optional union attribute in conditional #7884
• mypy infers conditional expression of Dict[str, object] to be object instead of union #7835
• Use Union for tuple fallback? #7616
• Wrong type inference (object instead of List) #6968
• Union types for dict keys are not inferred in union context #6079
• Conditional expression in "with" statement inferred as 'object', lacking __enter__ and __exit__ #5512
• False exception error in raise statement with a ternary conditional operator #5128
• Constrained TypeVar fails to narrow on left hand side of conditional expression #4134
• join is not associative in the presence of unions #3339

I believe that we should change this behavior.

If someone opens a PR to do this, it would be interesting to see what the results look like in mypy-primer and mypy's CI.

[A5rocks]

How will this interact with TypeForm vs Type?

Does this mean join(A,B) will still be accepted for Type[T]? Union[A, B] is not.

Nevermind: I was being confused. This doesn't matter I believe. (TypeForm is more about passing a type in, not about the type of an object)

[bluetech]

I noticed that this was tried for the specific case of conditional expressions (type of x if y else z) in 532f3fb (issue #3487) but was partly reverted in 94309a1 because it "caused many problems in internal Dropbox repos that we aren't ready to fix yet.".