Restore createAssigner (fixes compilation of merge and mergeWith) (#4101)

.internal/createAssigner.js

@@ -0,0 +1,36 @@
+import isIterateeCall from './isIterateeCall.js';
+
+/**
+ * Creates a function like `assign`.
+ *
+ * @private
+ * @param {Function} assigner The function to assign values.
+ * @returns {Function} Returns the new assigner function.
+ */
+function createAssigner(assigner) {
+  return (object, ...sources) => {
+    let index = -1;
+    let length = sources.length;
+    let customizer = length > 1 ? sources[length - 1] : undefined;
+    const guard = length > 2 ? sources[2] : undefined;
+
+    customizer = (assigner.length > 3 && typeof customizer == 'function')
+      ? (length--, customizer)
+      : undefined;
+
+    if (guard && isIterateeCall(sources[0], sources[1], guard)) {
+      customizer = length < 3 ? undefined : customizer;
+      length = 1;
+    }
+    object = Object(object);
+    while (++index < length) {
+      const source = sources[index];
+      if (source) {
+        assigner(object, source, index, customizer);
+      }
+    }
+    return object;
+  };
+}
+
+export default createAssigner;

.internal/isIterateeCall.js

@@ -0,0 +1,29 @@
+import isArrayLike from '../isArrayLike.js'
+import isIndex from './isIndex.js'
+
+/**
+ * Checks if the given arguments are from an iteratee call.
+ *
+ * @private
+ * @param {*} value The potential iteratee value argument.
+ * @param {*} index The potential iteratee index or key argument.
+ * @param {*} object The potential iteratee object argument.
+ * @returns {boolean} Returns `true` if the arguments are from an iteratee call,
+ *  else `false`.
+ */
+
+function isIterateeCall(value, index, object) {
+  if (!isObject(object)) {
+    return false;
+  }
+  const type = typeof index;
+  if (type == 'number'
+        ? (isArrayLike(object) && isIndex(index, object.length))
+        : (type == 'string' && index in object)
+      ) {
+    return eq(object[index], value);
+  }
+  return false;
+}
+
+export default isIterateeCall