def _fail_on_no_start_tls(self, req: "ClientRequest") -> None:
        """Raise a :py:exc:`RuntimeError` on missing ``start_tls()``.

        One case is that :py:meth:`asyncio.loop.start_tls` is not yet
        implemented under Python 3.6. It is necessary for TLS-in-TLS so
        that it is possible to send HTTPS queries through HTTPS proxies.

        This doesn't affect regular HTTP requests, though.
        """
        if not req.is_ssl():
            return

        proxy_url = req.proxy
        assert proxy_url is not None
        if proxy_url.scheme != "https":
            return

        self._check_loop_for_start_tls()
Raise a :py:exc:`RuntimeError` on missing ``start_tls()``.

        One case is that :py:meth:`asyncio.loop.start_tls` is not yet
        implemented under Python 3.6. It is necessary for TLS-in-TLS so
        that it is possible to send HTTPS queries through HTTPS proxies.

        This doesn't affect regular HTTP requests, though.
        
lib/aiohttp/connector.py