Hey@sethdumaguin - sorry it took so long for folks to get back to you.

I'm not precisely sure what's going on here, but I wonder if this is an issue where TypeScript can't appropriately infer the type from thetypes.reference call. I was able to remove the linting error by making these changes:

import { types } from "mobx-state-tree";const Base = types.model("Base", { name: "" }).actions((self) => ({  setName: (name: string) => {    self.name = name;  }}));const Foo = types.compose(  "Foo",  Base,  types.model({    type: "Foo"  }));const Bar = types.compose(  "Bar",  Base,  types.model({    id: types.identifier,    type: "Bar"  }));const Tree = types  .model("Tree", {    baz: types.union(types.reference(Bar), Foo) as typeof Bar | typeof Foo  })  .actions((self) => ({    changeName: () => {      self.baz.setName("foo");    }  }));

Here's the code sandbox:https://codesandbox.io/s/mobx-state-tree-todolist-forked-0lwr9n?file=/index.ts

The only change is explicitly usingas, like so:

baz:types.union(types.reference(Bar),Foo)astypeofBar|typeofFoo

I'm not sure if this is the fully correct way to do it, and I definitely don't love having to useas, but I do wonder if there's a limitation on TypeScript inferences, and maybe you just need to help the compiler a little bit here.

Is that helpful? Did you end up finding some other solution since posting this? Would love to get a sense of where you landed on this one.