Fuzzy type policies don't support type-level merge functions

[amannn]

Issue Description

I tried out fuzzy type policies in an app of mine and noticed that type-level merge functions don't work correctly. That means, at least an error is printed, the result looks ok (however it also does when I don't specify any type policies).

Link to Reproduction

https://codesandbox.io/p/devbox/apollo-client-cache-merge-svk8yx?file=%2Fsrc%2Findex.jsx%3A7%2C1

Reproduction Steps

1. Go to reproduction
2. See console output (an error is printed)

See the cache configuration in index.jsx: Depending on if fuzzy type matching is used or not, the merge function will be applied.

---

Related: #10639

[amannn]

Oh, I just noticed #6901 mentions possibleTypes while my reproduction uses typePolicies. I came here after this tweet by @phryneas. Can type policies also be fuzzy?

[bignimbus]

Hi @amannn 👋🏻 thanks for opening this issue! This area of the library is not something the current team has dealt with a lot so give us some time to get some recommendations together for you. @benjamn did recently write this very helpful spec though, which might help move things forward for you:

apollo-client/src/cache/inmemory/__tests__/policies.ts

Lines 664 to 687 in a604ac3

	it("typePolicies can be inherited from supertypes with fuzzy possibleTypes", () => {
	const cache = new InMemoryCache({
	possibleTypes: {
	EntitySupertype: [".*Entity"],
	},
	typePolicies: {
	Query: {
	fields: {
	coworkers: {
	merge(existing, incoming) {
	return existing ? existing.concat(incoming) : incoming;
	},
	},
	},
	},
	// The point of this test is to ensure keyFields: ["uid"] can be
	// registered for all __typename strings matching the RegExp /.*Entity/,
	// without manually enumerating all of them.
	EntitySupertype: {
	keyFields: ["uid"],
	},
	},
	});

[amannn]

Oh right, thank you so much!

I can confirm that this fixes my reproduction:

  new InMemoryCache({
    typePolicies: {
      Page: {
        keyFields: false,
        merge: true,
      },
    },
    possibleTypes: {
      Page: ["^.*Page$"],
    },
  }),

Working example with Page interface.

[github-actions]

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