Conditional types expand variants

[ppham27]

TypeScript Version: 3.0.0-dev.20180609

Search Terms: conditional types, type inference, union, boolean

Code

type Wrap<V = undefined> = V extends {} ? {value: V} : {};
type BooleanWrap = Wrap<boolean>;
const coinFlip: BooleanWrap = { value: Math.random() > 0.5 };

Expected behavior: The assignment to coinFlip should work.

Actual behavior: I get the compiler error:

index.ts:3:7 - error TS2322: Type '{ value: boolean; }' is not assignable to type '{ value: false; } | { value: true; }'.
  Type '{ value: boolean; }' is not assignable to type '{ value: true; }'.
    Types of property 'value' are incompatible.
      Type 'boolean' is not assignable to type 'true'.

3 const coinFlip: BooleanWrap = { value: Math.random() > 0.5 };

The type of BooleanWrap should ideally be resolved to { value: boolean; }, but it is resolved to { value: false; } | { value: true; }. The same thing happens with any type union.

Playground Link: https://www.typescriptlang.org/play/index.html#src=type%20Wrap%3CV%20%3D%20undefined%3E%20%3D%20V%20extends%20%7B%7D%20%3F%20%7Bvalue%3A%20V%7D%20%3A%20%7B%7D%3B%0Atype%20BooleanWrap%20%3D%20Wrap%3Cboolean%3E%3B%0Aconst%20coinFlip%3A%20BooleanWrap%20%3D%20%7B%20value%3A%20Math.random()%20%3E%200.5%20%7D%3B

Related Issues: #24085

[weswigham]

@ppham27

type Wrap<V = undefined> = [V] extends [{}] ? {value: V} : {};
type BooleanWrap = Wrap<boolean>;
const coinFlip: BooleanWrap = { value: Math.random() > 0.5 };

conditional types are distributive by default. To disable that, simply make the checked type not-a-bare-type-parameter, as above.

[RyanCavanaugh]

@weswigham beat me by seconds. We should seed a StackOverflow question on this

[ppham27]

This works for me. Thanks! Sorry for the bother. Is there some documentation that I'm missing about the bracket notation?

[RyanCavanaugh]

It's just the usual tuple syntax; any other wrapping of the type parameter would work, e.g. { foo: V }

[RyanCavanaugh]

Oh and I should add, all types extend {}, you almost certainly wanted to use object there

[ppham27]

As an aside, I had to negate the predicate:

type Wrap<V = undefined> = [V] extends [undefined] ? {} : {value: V} ;

[V] extends [{}] evaluates to true when V = undefined.

I don't want object because [V] extends [object] does not evaluate to true when V = boolean.