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Use NoInfer in GetTagMetadata to prevent inference widening #1418

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Use NoInfer in GetTagMetadata to prevent inference widening #1418

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2 changes: 1 addition & 1 deletion source/tagged.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -95,7 +95,7 @@ const parsed = parse(x); // The type of `parsed` is { hello: string }

@category Type
*/
export type GetTagMetadata<Type extends Tag<TagName, unknown>, TagName extends PropertyKey> = Type[typeof tag][TagName];
export type GetTagMetadata<Type extends Tag<NoInfer<TagName>, unknown>, TagName extends PropertyKey> = Type[typeof tag][TagName];

/**
Get the untagged portion of a tagged type created with `Tagged`.
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5 changes: 5 additions & 0 deletions test-d/opaque.ts
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Expand Up @@ -126,6 +126,11 @@ expectType<never>(undefined as unknown as GetTagMetadata<UrlString, 'URL'>);
type JsonOf<T> = Tagged<string, 'JSON', T>;
expectType<number>(JSON.parse('43') as GetTagMetadata<JsonOf<number>, 'JSON'>);

// GetTagMetadata: NoInfer prevents TagName widening from Type's structure (issue #848)
type MultiTagged = Tagged<Tagged<string, 'TagA', number>, 'TagB', boolean>;
expectType<number>(undefined as unknown as GetTagMetadata<MultiTagged, 'TagA'>);
expectType<boolean>(undefined as unknown as GetTagMetadata<MultiTagged, 'TagB'>);

// It's a type error to try to get the metadata for a tag that doesn't exist on a type.
// @ts-expect-error
const _a = '' as GetTagMetadata<UrlString, 'NonExistentTag'>;
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