fix(index.d.ts): correct type definition for SchemaType#cast()

Fix #9980

index.d.ts

@@ -2535,8 +2535,8 @@ declare module 'mongoose' {
     /** Attaches a getter for all instances of this schema type. */
     static get(getter: (value: any) => any): void;
 
-    /** Get/set the function used to cast arbitrary values to this type. */
-    cast(caster: (v: any) => any): (v: any) => any;
+    /** Cast `val` to this schema type. Each class that inherits from schema type should implement this function. */
+    cast(val: any, doc: Document<any>, init: boolean): any;
 
     /** Sets a default value for this SchemaType. */
     default(val: any): any;

lib/schematype.js

@@ -190,6 +190,19 @@ SchemaType.prototype.castFunction = function castFunction(caster) {
   return this._castFunction;
 };
 
+/**
+ * The function that Mongoose calls to cast arbitrary values to this SchemaType.
+ *
+ * @param {Object} value value to cast
+ * @param {Document} doc document that triggers the casting
+ * @param {Boolean} init
+ * @api public
+ */
+
+SchemaType.prototype.cast = function cast() {
+  throw new Error('Base SchemaType class does not implement a `cast()` function');
+};
+
 /**
  * Sets a default option for this schema type.
  *