I tried to follow the example from here to get http version 2 running. The error belongs to Cowboy but I dont understand it and I think its because I know too little about the OTP.
I tried to use as key both handler or adapter keys in dev.exs
https: [port: 4000, keyfile: "priv/server.key", certfile: "priv/server.pem"],
adapter: Phoenix.Endpoint.Cowboy2Handler,
** (Mix) Could not start application thttps: Thttps.Application.start(:normal, []) returned an error: shutdown: failed to start child: ThttpsWeb.Endpoint
** (EXIT) an exception was raised:
** (UndefinedFunctionError) function Phoenix.Endpoint.Cowboy2Handler.child_spec/3 is undefined or private
(phoenix) Phoenix.Endpoint.Cowboy2Handler.child_spec(:https, ThttpsWeb.Endpoint, [port: 4000, otp_app: :thttps, keyfile: "priv/server.key", certfile: "priv/server.pem"])