join is not associative in the presence of unions

[ilevkivskyi]

While working on #3322 I have found this problem:

lst1 = [1, 'a', None]
lst2 = [1, None, 'a']

reveal_type(lst1) # builtins.list[builtins.object*]
reveal_type(lst2) # builtins.list[Union[builtins.str, builtins.int, builtins.None]]

This turns out to be the cause of spurious --strict-optional errors I mentioned there.

[JukkaL]

It looks like joins should only produce unions if one of the operands is a union with more than one non-None item. Examples:

• join(Union[int, None], str) would be Union[object, None] which would get simplified into object.
• join(Union[int, bytes, None], str) would be Union[int, bytes, str, None].

[hauntsaninja]

Another example (which doesn't use None)

def check(l_int: list[int], l_str: list[str], l_union: list[str] | list[bytes]):
    reveal_type([l_int, l_str])
    reveal_type([l_int, l_str, l_union])
    reveal_type([l_int, l_union, l_str])

[ikonst]

Maybe worth clarifying (in the original post) what's the desired outcome? I assume builtins.list[Union[builtins.str, builtins.int, builtins.None]] but wanted to make sure.