Union types for dict keys are not inferred in union context

[bryanforbes]

mypy: 0.650
Python: 3.7.1

I'm not sure if this is a bug or a new feature, however the following code is unexpected (in my mind, especially coming from TypeScript):

d = {"1": 2, 3: 4}
reveal_type(d)  # builtins.dict[builtins.object*, builtins.int*]

I would have expected mypy to infer the type of d as builtins.dict[Union[builtins.str*, builtins.int*], builtins.int*].

[ilevkivskyi]

The situation is more complex than that. The fact mypy doesn't infer unions for joins is expected. What is not expected is that mypy doesn't infer unions in a union context. Here are better examples:

x: List[Union[int, str]] = [42, 'hi']  # OK
y: Union[List[Union[int, str]], List[int]] = [42, 'hi'] # Incompatible types in assignment because of `List[object]`

Our treatment of unions for inference is not very principled, but doing it right might be hard. And TBH I don't have any clear idea.

A bit more details: using the external context we infer constrains against every item of the outer union that leads to unsatisfiable constraints. Then falling back to inner context causes us to infer a join which is an object. Note that putting all constraints into the same bin will not help here.