dict literals inferred like immutable Mapping

Bug Report

dict literal behaves as though it is inferred to be (immutable) Mapping

To Reproduce

class OrmBase:
    pass

class Orm1(OrmBase):
    pass

class Orm1Prime(OrmBase):
    pass

class Orm2(OrmBase):
    pass

class Orm2Prime(OrmBase):
    pass

def func(orm: Union[Orm1, Orm2]):
    orm_mapper = {
        Orm1: Orm1Prime, 
        Orm2: Orm2Prime,
    }
    mapped: Union[Type[Orm1Prime], Type[Orm2Prime]] = orm_mapper[orm.__class__]
    # Incompatible types in assignment (expression has type "Type[OrmBase]",
    # variable has type "Union[Type[Orm1Prime], Type[Orm2Prime]]") [E]
    return mapped

Expected Behavior

dict is a MutableMapping, which is invariant with respect to value types. The Union on mapped should be acceptable here. (Pyright has no issue with this.)

Actual Behavior

The Union is rejected, apparently because the parent class is expected (see the error message above).

Adding a plain dict annotation resolves the error:

orm_mapper: dict = { ...

I believe this resolves the error by telling Mypy that this is a MutableMapping with invariant VT and not a Mapping with covariant VT_co. typing's definitions of both

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
plugins = sqlalchemy.ext.mypy.plugin
ignore_missing_imports = True

[mypy-sqlalchemy.*]

• Python version used: 3.9.6
• Operating system and version: Fedora 34 (Workstation)

まず、提供された再現コードを mypy 0.910 で実行し、アノテーションのない dict リテラルの推論された型と、明示的にアノテーションされたバージョンの型を比較します。dict リテラルの推論経路を追跡し、型安全性を弱めることなく代入が期待される union を受け入れることを確認します。再現コードが成功すれば issue は解決です。