[4.10.0][Regression] failure when type checking with exactOptionalPropertyTypes

[AviVahl]

mkdir show-issue
cd show-issue
npm init -y
npm i typescript type-fest -D
npx tsc --init --skipLibCheck false --target es2022 --exactOptionalPropertyTypes
echo "import 'type-fest';" > test.ts
npx tsc --noEmit

See the following error:

node_modules/type-fest/source/merge-deep.d.ts:292:50 - error TS2344: Type '{ [KeyType in keyof ({ [Key in keyof PickIndexSignature<Options> as Key extends never ? never : Key]: PickIndexSignature<Options>[Key]; } & PickIndexSignature<...> & { [Key in keyof OmitIndexSignature<...> as Key extends "spreadTopLevelArrays" ? never : Key]: OmitIndexSignature<...>[Key]; } & OmitIndexSignature<...>...' does not satisfy the constraint '{ arrayMergeMode?: ArrayMergeMode; recurseIntoArrays?: boolean; spreadTopLevelArrays?: boolean; }'.
  Types of property 'arrayMergeMode' are incompatible.
    Type 'Options["arrayMergeMode"]' is not assignable to type 'ArrayMergeMode'.
      Type 'ArrayMergeMode | undefined' is not assignable to type 'ArrayMergeMode'.
        Type 'undefined' is not assignable to type 'ArrayMergeMode'.

292     ? MergeDeepArrayOrTuple<Destination, Source, Merge<Options, {spreadTopLevelArrays: false}>>
                                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Found 1 error in node_modules/type-fest/source/merge-deep.d.ts:292

Saw first in wixplosives/core3-playground#175

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[sindresorhus]

// @Emiyaaaaa

[Emiyaaaaa]

// @Emiyaaaaa

#802 I was tested that it's working after the revert, we can revert first.

exactOptionalPropertyTypes is false in type-fest's tsconfig, should we also support exactOptionalPropertyTypes: true?

[Emiyaaaaa]

I found some interesting thing:

Seem like Merge<T, T> is {a?: T['a']} and certainty T['a'] is string | undefined,
then string | undefined is not assignable to type 'string' which is a in A, so ts throw an error

So the basic problem is that Merge<T, T> is {a?: T['a']} when T is a generic type, this maybe is a feature or rules of typescript I don't know.

In type-fest@4.9.0, Merge used EnforceOptional to convert undefined to ? and remove undefined, but in fact this is a wrong action and people reported the issue. So I removed EnforceOptional in 4.10.0.

If we need to fix tsc error when exactOptionalPropertyTypes: true, I think we can only resolve bug case-by-case like this

How about you think? @sindresorhus

[sindresorhus]

exactOptionalPropertyTypes is false in type-fest's tsconfig, should we also support exactOptionalPropertyTypes: true?

Yes, we should add it to our config.

[AviVahl]

Thanks! ❤️