types: allow calling SchemaType.cast() without parent and init parameters

test/schema.test.js

@@ -3258,4 +3258,24 @@ describe('schema', function() {
 
     await q;
   });
+
+  it('supports casting object to subdocument (gh-14748) (gh-9076)', function() {
+    const nestedSchema = new Schema({ name: String });
+    nestedSchema.methods.getAnswer = () => 42;
+
+    const schema = new Schema({
+      arr: [nestedSchema],
+      singleNested: nestedSchema
+    });
+
+    // Cast to doc array
+    let subdoc = schema.path('arr').cast([{ name: 'foo' }])[0];
+    assert.ok(subdoc instanceof mongoose.Document);
+    assert.equal(subdoc.getAnswer(), 42);
+
+    // Cast to single nested subdoc
+    subdoc = schema.path('singleNested').cast({ name: 'bar' });
+    assert.ok(subdoc instanceof mongoose.Document);
+    assert.equal(subdoc.getAnswer(), 42);
+  });
 });

test/types/schema.test.ts

@@ -1,5 +1,6 @@
 import {
   DefaultSchemaOptions,
+  HydratedArraySubdocument,
   HydratedSingleSubdocument,
   Schema,
   Document,
@@ -1555,3 +1556,22 @@ function gh14696() {
   });
 
 }
+
+function gh14748() {
+  const nestedSchema = new Schema({ name: String });
+
+  const schema = new Schema({
+    arr: [nestedSchema],
+    singleNested: nestedSchema
+  });
+
+  const subdoc = schema.path('singleNested')
+    .cast<HydratedArraySubdocument<{ name: string }>>({ name: 'bar' });
+  expectAssignable<{ name: string }>(subdoc);
+
+  const subdoc2 = schema.path('singleNested').cast({ name: 'bar' });
+  expectAssignable<{ name: string }>(subdoc2);
+
+  const subdoc3 = schema.path<Schema.Types.Subdocument<{ name: string }>>('singleNested').cast({ name: 'bar' });
+  expectAssignable<{ name: string }>(subdoc3);
+}

types/schematypes.d.ts

@@ -220,7 +220,8 @@ declare module 'mongoose' {
     OptionsConstructor: SchemaTypeOptions<T>;
 
     /** 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, prev?: any, options?: any): any;
+    cast(val: any, doc?: Document<any>, init?: boolean, prev?: any, options?: any): any;
+    cast<ResultType>(val: any, doc?: Document<any>, init?: boolean, prev?: any, options?: any): ResultType;
 
     /** Sets a default value for this SchemaType. */
     default(val: any): any;
@@ -443,7 +444,7 @@ declare module 'mongoose' {
         defaultOptions: Record<string, any>;
       }
 
-      class Subdocument extends SchemaType implements AcceptsDiscriminator {
+      class Subdocument<DocType = unknown> extends SchemaType implements AcceptsDiscriminator {
         /** This schema type's name, to defend against minifiers that mangle function names. */
         static schemaName: string;
 
@@ -455,6 +456,8 @@ declare module 'mongoose' {
 
         discriminator<T, U>(name: string | number, schema: Schema<T, U>, value?: string): U;
         discriminator<D>(name: string | number, schema: Schema, value?: string): Model<D>;
+
+        cast(val: any, doc?: Document<any>, init?: boolean, prev?: any, options?: any): HydratedSingleSubdocument<DocType>;
       }
 
       class String extends SchemaType {