FULL PRODUCT VERSION :
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.16299.309]

A DESCRIPTION OF THE PROBLEM :
Send a HTTPS request though a proxy connection based on HTTP_2 protocol throws ClassCastException

"
java.lang.ClassCastException: jdk.incubator.httpclient/jdk.incubator.http.SSLTunnelConnection cannot be cast to jdk.incubator.httpclient/jdk.incubator.http.AsyncConnection
"

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the source code below

ACTUAL -
java.lang.ClassCastException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.ClassCastException: jdk.incubator.httpclient/jdk.incubator.http.SSLTunnelConnection cannot be cast to jdk.incubator.httpclient/jdk.incubator.http.AsyncConnection

at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.<init>(Http2Connection.java:270)
at jdk.incubator.httpclient/jdk.incubator.http.Http2ClientImpl.getConnectionFor(Http2ClientImpl.java:108)
at jdk.incubator.httpclient/jdk.incubator.http.ExchangeImpl.get(ExchangeImpl.java:86)
at jdk.incubator.httpclient/jdk.incubator.http.Exchange.establishExchange(Exchange.java:257)
at jdk.incubator.httpclient/jdk.incubator.http.Exchange.responseImpl0(Exchange.java:268)
at jdk.incubator.httpclient/jdk.incubator.http.Exchange.responseImpl(Exchange.java:245)
at jdk.incubator.httpclient/jdk.incubator.http.Exchange.response(Exchange.java:121)
at jdk.incubator.httpclient/jdk.incubator.http.MultiExchange.response(MultiExchange.java:154)
at jdk.incubator.httpclient/jdk.incubator.http.HttpClientImpl.send(HttpClientImpl.java:234)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
somewhatHttpClient.send(
HttpRequest
.newBuilder()
.uri(URI.create("https://www.google.com"))
.GET()
.version(HttpClient.Version.HTTP_2)
.build(),
HttpResponse.BodyHandler.asString()
);
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
DROP module jdk.incubator.httpclient