See the mail thread beginning here:
http://mail.openjdk.java.net/pipermail/jigsaw-dev/2018-December/014076.html
and reference on StackOverflow:
https://stackoverflow.com/q/51094274/4611488

Extract from initial post:
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The question can be simplified like this:
//---
import javax.xml.transform.Transformer;

public class Test {
    Transformer transformer;
}
//---

Which of the following compiler invocations should accept / reject the program:

$ javac Test.java
$ javac -classpath xml-apis-1.4.01.jar Test.java
$ javac -classpath xml-apis-1.4.01.jar --limit-modules=java.base,java.xml Test.java
$ javac -classpath xml-apis-1.4.01.jar --limit-modules=java.base Test.java :
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Extract of response from [~abuckley]
 
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I agree that only the first and last invocations should succeed.

tl;dr A package split between the classpath and the system image is just as bad as a package split between two modules on the modulepath.
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See attached test.