Add changeset

.changeset/moody-lobsters-listen.md

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+---
+"@apollo/client": major
+---
+
+The `Cache.DiffResult<T>` type is now a union type with better type safety for both complete and partial results. Checking `diff.complete` will now narrow the type of `result` depending on whether the value is `true` or `false`.
+
+When `true`, `diff.result` will be a non-null value equal to the `T` generic type. When `false`, `diff.result` now reports `result` as `DeepPartial<T> | null` indicating that fields in the result may be missing (`DeepPartial<T>`) or empty entirely (`null`).

.changeset/tall-cups-suffer.md

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+---
+"@apollo/client": major
+---
+
+`cache.diff` now returns `null` instead of an empty object (`{}`) when `returnPartialData` is `true` and the result is empty.
+
+If you use `cache.diff` directly with `returnPartialData: true`, you will need to check for `null` before accessing any other fields on the `result` property. A non-null value indicates that at least one field was present in the cache for the given query document.

.changeset/yellow-cats-judge.md

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+---
+"@apollo/client": major
+---
+
+`cache.diff` no longer throws when `returnPartialData` is set to `false` without a complete result. Instead, `cache.diff` will return `null` when it is unable to read a full cache result.
+
+If you use `cache.diff` directly with `returnPartialData: false`, remove the `try`/`catch` block and replace with a check for `null`.