// Helper type. Not useful on its own.
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type Without<FirstType, SecondType> = {[KeyType in Exclude<keyof FirstType, keyof SecondType>]?: never};
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/**
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Create a type that has mutually exclusive keys.
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This type was inspired by [this comment](https://github.com/Microsoft/TypeScript/issues/14094#issuecomment-373782604).
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This type works with a helper type, called `Without`. `Without<FirstType, SecondType>` produces a type that has only keys from `FirstType` which are not present on `SecondType` and sets the value type for these keys to `never`. This helper type is then used in `MergeExclusive` to remove keys from either `FirstType` or `SecondType`.
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@example
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```
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import {MergeExclusive} from 'type-fest';
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interface ExclusiveVariation1 {
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exclusive1: boolean;
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}
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interface ExclusiveVariation2 {
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exclusive2: string;
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}
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type ExclusiveOptions = MergeExclusive<ExclusiveVariation1, ExclusiveVariation2>;
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let exclusiveOptions: ExclusiveOptions;
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exclusiveOptions = {exclusive1: true};
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//=> Works
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exclusiveOptions = {exclusive2: 'hi'};
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//=> Works
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exclusiveOptions = {exclusive1: true, exclusive2: 'hi'};
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//=> Error
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```
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*/
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export type MergeExclusive<FirstType, SecondType> =
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(FirstType | SecondType) extends object ?
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(Without<FirstType, SecondType> & SecondType) | (Without<SecondType, FirstType> & FirstType) :
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FirstType | SecondType;
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