Type 'boolean' is not assignable to type 'true' when constructing discriminated union variant

[deadalusai]

TypeScript Version: 2.0.2 RC

Code

export interface Some<T> {
 is_some: true;
 value: T;
}

export interface None { 
 is_some: false;
}

// fails typecheck
var a: Option<number> = { is_some: true, value: 1 };

Expected behavior:

Expected to pass typechecking.

Actual behavior:

The following error is raised:

src/option.ts(13,5): error TS2322: Type '{ is_some: boolean; value: number; }' is not assignable to type 'Option<number>'.
 Type '{ is_some: boolean; value: number; }' is not assignable to type 'Some<number>'.
 Types of property 'is_some' are incompatible.
 Type 'boolean' is not assignable to type 'true'.

Adding a cast of the boolean value to type true appears to fix the error:

// working example
var a: Option<number> = { is_some: <true> true, value: 1 };

I encountered this problem when experimenting with the following function. A version of the code which uses strings as the discriminator works without casts. Another version which uses value: null as the discriminator in the None case also works.

// failing example
export function map<T, N>(opt: Option<T>, fn: (value: T) => N): Option<N> {
 if (opt.is_some) {
 return { is_some: true, value: fn(opt.value) };
 }
 else {
 return { is_some: false }; 
 }
}

[yortus]

Duplicate of #10432. The PR in #10577 should fix this.

[deadalusai]

Referenced issue looks to be identical - even the Same code sample. Closing as dupe.

[tcoopman]

I have something similar in typescript 2.4.2, but it looks like it's only when I have return in a promise. Casting the boolean type to true or false fixes the problem.