Generic type falling back to object - possibly related to type[ Union ]?

[JakeSummers]

Bug Report

The following code produces a type error but it shouldn't.

• Gist URL: https://gist.github.com/mypy-play/b4d24f9b14f909a7d6550ff738cd4fb1
• Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&gist=b4d24f9b14f909a7d6550ff738cd4fb1

from dataclasses import dataclass
from typing import TypeVar, Mapping, reveal_type


@dataclass
class A:
    foo: str = "a"

@dataclass
class B:
    bar: str = "b"

lookup_table: Mapping[str, type[A] | type[B]] = {
    "a": A,
    "b": B
}

reveal_type(lookup_table)  # note: Revealed type is "typing.Mapping[builtins.str, Union[type[simple.A], type[simple.B]]]"

T = TypeVar("T")

def load(lookup_table: Mapping[str, type[T]], lookup_key:str) -> T:
    con: type[T] = lookup_table[lookup_key]
    instance: T = con()
    return instance

example_a: A | B = load(lookup_table, "a")  # error: Incompatible types in assignment (expression has type "object", variable has type "A | B")
print(example_a)

Error:

error: Incompatible types in assignment (expression has type "object", variable has type "A | B")  [assignment]

Your Environment

• Mypy version used: 1.13.0 (latest on mypy Playground)
• Mypy command-line flags: None.
• Python version used: 3.11, 3.12, 3.13

[sterliakov]

Duplicates #12115