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

[seansfkelley]

TypeScript Version: v3.5.1

Search Terms: union partial type assignable structure

Code

// 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>;

Expected behavior: Line 21, the structurally-compatible assignment, compiles, without a workaround. Using the Expand type has no effect on assignability.

Actual behavior: Line 21 does not compile, stating

Type 'Pick<Either, "key" | "type">' is not assignable to type 'PartialEither'.
 Type 'Pick<Either, "key" | "type">' is not assignable to type '_PartialEither<KeyValue>'.
 Type 'Pick<Either, "key" | "type">' is not assignable to type 'Pick<KeyValue, "key" | "type">'.
 Types of property 'type' are incompatible.
 Type '"key" | "key-value"' is not assignable to type '"key-value"'.
 Type '"key"' is not assignable to type '"key-value"'.

Using the Expand helper type workaround fixes the assignability problem.

Playground Link: link

Related Issues: #18538, #19927

[jack-williams]

My answer was a mess!

The reason why Expand works is probably because of this #30779; both approaches do not work pre 3.5. It seems that PartialEither as defined is not considered a discriminated union in that logic. TypeScript clearly understands that PartialEither is a discriminated union though, evidenced by:

if (either.type === "key-value") {
 either // narrowed
}

Smaller repro:

type X = 
 | { x: 'x', y: number } & { y: number } 
 | { x: 'y', y: number, z?: boolean } & { y: number }

// error
const x: X = 4 as any as { x: 'x' | 'y', y: number };

type Y = 
 | { x: 'x', y: number } 
 | { x: 'y', y: number, z?: boolean }

// no error
const y: Y = 4 as any as { x: 'x' | 'y', y: number };