Return conditional expr incorrectly complains about union

[gvanrossum]

This gives an error:

def foo(a: bool) -> Dict[str, Union[str, int]]:
    return {'a': 'b'} if a else {}  # E: Incompatible return value type: expected builtins.dict[builtins.str, Union[builtins.str, builtins.int]], got builtins.dict[builtins.str, builtins.str]

However this rewrite is fine:

def foo(a: bool) -> Dict[str, Union[str, int]]:
    if a:
        return {'a': 'b'}
    else:
        return {}

[gvanrossum]

It gets weirder. Try this:

def foo(a: bool) -> Dict[str, Union[str, int]]:
    return {'a': 'b'} if a else {'a': 0}

This gives the same error as above PLUS:

x.py:4: error: List item 0 has incompatible type "Tuple[str, int]"

That error also shows up for just this (at the top level):

{'a': 'b'} if True else {'a': 0}

[gvanrossum]

The latter issue may be a red herring; it's because if-else expressions aren't balanced, and it looks weird because the dict literals are translated to dict() calls. A similar error appears for a simpler case:

[''] if True else [0]  # E: List item 0 has incompatible type "int"

[ddfisher]

Looks similar to #1094

[gvanrossum]

Yeah, they're duplicates. We could probably fix a few common examples without doing full unification.