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Fix TaggedUnion constraint to allow documented generic usage #1430

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Fix TaggedUnion constraint to allow documented generic usage #1430

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4 changes: 2 additions & 2 deletions source/tagged-union.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ Use-case: A shorter way to declare tagged unions with multiple members.
```
import type {TaggedUnion} from 'type-fest';

type Tagged<Fields extends Record<string, Record<string, unknown>>> = TaggedUnion<'type', Fields>;
type Tagged<Fields extends Record<string, unknown>> = TaggedUnion<'type', Fields>;

// The TaggedUnion utility reduces the amount of boilerplate needed to create a tagged union with multiple members, making the code more concise.
type EventMessage = Tagged<{
Expand Down Expand Up @@ -45,7 +45,7 @@ type ManualEventMessage =
*/
export type TaggedUnion<
TagKey extends string,
UnionMembers extends Record<string, Record<string, unknown>>,
UnionMembers extends Record<string, unknown>,
> = {
[Name in keyof UnionMembers]: {[Key in TagKey]: Name} & UnionMembers[Name];
}[keyof UnionMembers];
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15 changes: 15 additions & 0 deletions test-d/tagged-union.ts
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Expand Up @@ -28,3 +28,18 @@ const failsToo = {

expectNotAssignable<Union>(fails);
expectNotAssignable<Union>(failsToo);

// Issue #1211: `TaggedUnion` should be usable through a generic wrapper whose
// members are constrained as `Record<string, unknown>`, as shown in the docs.
type Tagged<Fields extends Record<string, unknown>> = TaggedUnion<'type', Fields>;

type EventMessage = Tagged<{
OpenExternalUrl: {url: string; id: number};
ToggleBackButtonVisibility: {visible: boolean};
}>;

expectAssignable<EventMessage>({type: 'OpenExternalUrl', url: 'https://example.com', id: 1});
expectAssignable<EventMessage>({type: 'ToggleBackButtonVisibility', visible: true});

// The discriminant still narrows the field types correctly.
expectNotAssignable<EventMessage>({type: 'ToggleBackButtonVisibility', visible: 'yes'});