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Type 'boolean' is not assignable to type 'true' when constructing discriminated union variant #10657
Description
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 };
}
}
Activity
yortus commentedon Sep 1, 2016
Duplicate of #10432. The PR in #10577 should fix this.
deadalusai commentedon Sep 1, 2016
Referenced issue looks to be identical - even the Same code sample. Closing as dupe.
tcoopman commentedon Jul 21, 2017
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.