Union of partial types: common super type of all union members is not assignable to union type #33243

// Data types

interface Key {
  type: "key";
  key: string;
}

interface KeyValue {
  type: "key-value";
  key: string;
  value: string;
}

type Either = Key | KeyValue;

type Common = Pick<Either, "type" | "key">;

// Test cases

// Structurally-compatible assignment
const either: PartialEither = null as any as Common;

// Type-guarding
if (either.type === "key-value") {
  either.value;
}

// Problematic type definitions

// Using this intermediate type so I can still rely on 'type' as the discriminant property of
// the PartialEither type. The naive
// Pick<Either, "type" | "key"> & Partial<Omit<Either, "type" | "key">>
// would lose the relationship between `type` and shape, causing the type guard above to fail.
type _PartialEither<T extends Either> = Pick<T, "type" | "key"> & Partial<Omit<T, "type" | "key">>;

type PartialKey = _PartialEither<Key>;
type PartialKeyValue = _PartialEither<KeyValue>;

type PartialEither = PartialKey | PartialKeyValue;

// This type seems to force the compiler to reinterpret the partial types into a slightly
// different representation that's logically equivalent but properly assignable. From:
// https://stackoverflow.com/questions/57780109/union-of-partial-types-in-typescript-cant-be-type-narrowed
type Expand<T> = T extends infer O ? { [K in keyof O]: O[K] } : never;
// Change the definition to this version to see the test cases above work.
// type PartialEither = Expand<PartialKey | PartialKeyValue>;