Undocumented = operator syntax in Generic Constraining

In the declaration file for Fastify (https://github.com/fastify/fastify/blob/master/fastify.d.ts) there is some TypeScript code I do not recognize and cannot find another example of in the TypeScript documentation:

declare function fastify<
  HttpServer extends (http.Server | http2.Http2Server) = http.Server,
  HttpRequest extends (http.IncomingMessage | http2.Http2ServerRequest) = http.IncomingMessage,
  HttpResponse extends (http.ServerResponse | http2.Http2ServerResponse) = http.ServerResponse
>(opts?: fastify.ServerOptions): fastify.FastifyInstance<HttpServer, HttpRequest, HttpResponse>;

I recognize the Generic constraint syntax with the extends keyword, but what about the = http.Server part? Is this like a default parameter or something?

Whatever this syntax means it should be better documented.

I tried googling for this syntax by searching

• typescript generic constraint
• typescript generic constraint assignment
• typescript generic defaults

まず Fastify の fastify.d.ts にあるジェネリック宣言を読み、Issue に記載されている TypeScript ドキュメントの検索結果と、その制約およびデフォルト構文を比較します。= http.Server の部分が何を意味するのかを説明し、適切な TypeScript ドキュメントに例または説明を追加します。新しく参加する人がこの構文を見つけて理解できるようになれば完了です。