Implement a naive concatPagination field policy function.

This pagination helper policy is flawed because it does not consider any
field arguments, but it seems useful for AC3 migration purposes, because
it does exactly what a typical fetchMore updateQuery function would do,
just a little more reliably.

src/cache/inmemory/policies.ts

@@ -186,7 +186,7 @@ export type FieldMergeFunction<TExisting = any, TIncoming = TExisting> = (
   // reasons discussed in FieldReadFunction above.
   incoming: SafeReadonly<TIncoming>,
   options: FieldFunctionOptions,
-) => TExisting;
+) => SafeReadonly<TExisting>;
 
 export const defaultDataIdFromObject = (
   { __typename, id, _id }: Readonly<StoreObject>,

src/utilities/index.ts

@@ -63,5 +63,6 @@ export {
 } from './graphql/transform';
 
 export {
+  concatPagination,
   offsetLimitPagination,
 } from './policies/pagination';

src/utilities/policies/pagination.ts

@@ -2,6 +2,22 @@ import { FieldPolicy, Reference } from '../../cache';
 
 type KeyArgs = FieldPolicy<any>["keyArgs"];
 
+// A very basic pagination field policy that always concatenates new
+// results onto the existing array, without examining options.args.
+export function concatPagination<T = Reference>(
+  keyArgs: KeyArgs = false,
+): FieldPolicy<T[]> {
+  return {
+    keyArgs,
+    merge(existing, incoming) {
+      return existing ? [
+        ...existing,
+        ...incoming,
+      ] : incoming;
+    },
+  };
+}
+
 // A basic field policy that uses options.args.{offset,limit} to splice
 // the incoming data into the existing array. If your arguments are called
 // something different (like args.{start,count}), feel free to copy/paste