test: add test for issue 1525

__tests__/core/1525.test.ts

@@ -0,0 +1,33 @@
+import { types, Instance } from "../../src/index"
+
+describe("1525. Model instance maybe fields becoming TypeScript optional fields when included in a types.union", () => {
+  it("does not throw a typescript error", () => {
+    const Model = types.model("myModel", {
+      foo: types.string,
+      bar: types.maybe(types.integer)
+    })
+
+    const Store = types.model("store", {
+      itemWithoutIssue: Model,
+      itemWithIssue: types.union(types.literal("anotherValue"), Model)
+    })
+
+    interface IModel extends Instance<typeof Model> {}
+
+    interface FunctionArgs {
+      model1: IModel
+      model2: IModel
+    }
+
+    const store = Store.create({
+      itemWithoutIssue: { foo: "works" },
+      itemWithIssue: { foo: "has ts error in a regression" }
+    })
+
+    const f = (props: FunctionArgs) => {}
+
+    const itemWithoutIssueModel = store.itemWithoutIssue
+    const itemWithIssueModel = store.itemWithIssue === "anotherValue" ? null : store.itemWithIssue
+    itemWithIssueModel && f({ model1: itemWithoutIssueModel, model2: itemWithIssueModel })
+  })
+})