Boolean type property access is not expanded into true | false in a conditional type.

[calebmer]

TypeScript Version: 3.5.1

Search Terms: boolean, property

Code

type MyOptions = {
  foo: boolean;
};

type MyType<Options extends MyOptions> = Options["foo"] extends true ? number : string;

declare const a: number | string;
const b: MyType<MyOptions> = a;

Expected behavior: This example should pass, MyType<MyOptions> should be number | string.

Actual behavior: This example fails, MyType<MyOptions> is false.

Playground Link: TypeScript Playground

Related Issues:

#30708 looks similarish. However, it is my understanding that nested union types will not be expanded when compared with a conditional. So [boolean] extends [true] would not be expanded to [true | false] extends [true] and would not be expanded to [true] | [false] extends [true].

I‘d expect this example to pass since we are explicitly putting the union (in this case boolean) under the conditional’s test.

[calebmer]

cc @mdgozza who discovered this, I helped make a minimal repro

[jack-williams]

This is the right behaviour because MyType is not a distributive conditional type; boolean extends true will always be false as the union is viewed atomically.

It needs to be written as:

type Dist<T> = T extends true ? number : string;
type MyType<Options extends MyOptions> = Dist<Options["foo"]>;

[fatcerberus]

Yep, this is what the documentation means when it says distribution happens over a “naked type parameter”. It’s also why you can easily disable distribution by writing the type as [T] extends [U] ? ...

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.