Dynamically .merge or .extend

[fehnomenal]

First of all thanks for this great library. I use it everywhere I can 👍

Lately I work with Rest APIs where the response shape depends on the request.

It is something like this

{
  "searchQuery": "...",
  "searchType": 0,
  "includeGenericArticles": true
}

The response will contain generic article information only if includeGenericArticles was given as true:

{
  "totalMatchingArticles": 1,
  "maxAllowedPage": 1,
  "articles": [
    {
      "articleNumber": "...",
      "genericArticles": [ {}, {} ]
    }
  ],
  "status": 200
}

At runtime I can express this easily by building the response schema dynamically:

const articleSchema = articleBaseSchema.extend(
  request.includeGenericArticles ? genericArticlesExtension.shape : {}
);

This works and now I want to get type-safety also at compile time, i.e.:

(await getArticles({ includeGenericArticles: true })).articles[0].genericArticles; // <-- works

(await getArticles({})).articles[0].genericArticles; // <-- does not work

It tried it with generics, conditional types and z.extendShape but it won't work:

type RequestIncludes = Pick<GetArticlesRequest, 'includeGenericArticles'>;

// I'm using this utility type here as there are many endpoints and fields that behaves this way
// and I want to establish a pattern accross the code base.
type AdditionalArticleShape<Req extends RequestIncludes> =
  Req['includeGenericArticles'] extends true
    ? typeof articleGenericArticlesSchemaExtension.shape
    : {};

const buildArticleShape = <Req extends RequestIncludes>(
  request: Req,
): z.extendShape<typeof articleBaseSchema.shape, AdditionalArticleShape<Req>> => {
  return z.objectUtil.mergeShapes(
    articleBaseSchema.shape,
    request.includeGenericArticles ? articleGenericArticlesSchemaExtension.shape : {}, // <-- type checking fails here
  );
};

// buildArticleShape works as expected
z.object(buildArticleShape({})).parse(null).genericArticles; // <-- does not work
z.object(buildArticleShape({ includeGenericArticles: true })).parse(null).genericArticles; // <-- does work

If I remove the ternary and use always the extension shape, it type checks.

Has anybody an idea how I can solve this problem?