Fix issue where a merge function that returns an incomplete result would not allow the client to refetch from network

.changeset/itchy-dodos-approve.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Fix a regression in [3.9.5](https://github.com/apollographql/apollo-client/releases/tag/v3.9.5) where a merge function that returned an incomplete result would not allow the client to refetch in order to fulfill the query.

src/core/QueryInfo.ts

@@ -211,22 +211,16 @@ export class QueryInfo {
   setDiff(diff: Cache.DiffResult<any> | null) {
     const oldDiff = this.lastDiff && this.lastDiff.diff;
 
-    // If we do not tolerate partial results, skip this update to prevent it
-    // from being reported. This prevents a situtuation where a query that
-    // errors and another succeeds with overlapping data does not report the
-    // partial data result to the errored query.
+    // If we are trying to deliver an incomplete cache result, we avoid
+    // reporting it if the query has errored, otherwise we let the broadcast try
+    // and repair the partial result by refetching the query. This check avoids
+    // a situation where a query that errors and another succeeds with
+    // overlapping data does not report the partial data result to the errored
+    // query.
     //
     // See https://github.com/apollographql/apollo-client/issues/11400 for more
     // information on this issue.
-    if (
-      diff &&
-      !diff.complete &&
-      !this.observableQuery?.options.returnPartialData &&
-      // In the case of a cache eviction, the diff will become partial so we
-      // schedule a notification to send a network request (this.oqListener) to
-      // go and fetch the missing data.
-      !(oldDiff && oldDiff.complete)
-    ) {
+    if (diff && !diff.complete && this.observableQuery?.getLastError()) {
       return;
     }