Slight tweak to merge function typing in fetchMore tests.

src/__tests__/fetchMore.ts

@@ -2,7 +2,7 @@ import { assign, cloneDeep } from 'lodash';
 import gql from 'graphql-tag';
 
 import { itAsync, mockSingleLink, subscribeAndCount } from '../testing';
-import { InMemoryCache, InMemoryCacheConfig } from '../cache';
+import { InMemoryCache, InMemoryCacheConfig, FieldMergeFunction } from '../cache';
 import { ApolloClient, NetworkStatus, ObservableQuery } from '../core';
 import { offsetLimitPagination, concatPagination } from '../utilities';
 
@@ -412,7 +412,8 @@ describe('fetchMore on an observable query', () => {
     const { merge } = groceriesFieldPolicy;
     groceriesFieldPolicy.merge = function (existing, incoming, options) {
       mergeArgsHistory.push(options.args);
-      return (merge as any).call(this, existing, incoming, options);
+      return (merge as FieldMergeFunction<any>).call(
+        this, existing, incoming, options);
     };
 
     const cache = new InMemoryCache({