Starting any webapp with weld (weld-servelt-shaded.jar) under Java 12 results in an exception. With Java 11 everything works fine. See attached detailed log file. I think the relevant part is the following:

Caused by: org.jboss.weld.exceptions.WeldException: WELD-001524: Unable to load proxy class for bean Implicit Bean [javax.enterprise.inject.Instance] with qualifiers [@Default] with class interface javax.enterprise.inject.Instance
at org.jboss.weld.bean.proxy.ProxyFactory.getProxyClass(ProxyFactory.java:378)
at org.jboss.weld.bean.builtin.AbstractFacadeBean.initializeAfterBeanDiscovery(AbstractFacadeBean.java:61)
at org.jboss.weld.bootstrap.ConcurrentBeanDeployer$AfterBeanDiscoveryInitializerFactory.doWork(ConcurrentBeanDeployer.java:111)
at org.jboss.weld.bootstrap.ConcurrentBeanDeployer$AfterBeanDiscoveryInitializerFactory.doWork(ConcurrentBeanDeployer.java:102)
at org.jboss.weld.executor.IterativeWorkerTaskFactory$1.call(IterativeWorkerTaskFactory.java:62)
at org.jboss.weld.executor.IterativeWorkerTaskFactory$1.call(IterativeWorkerTaskFactory.java:55)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:835)
Caused by: java.lang.NoClassDefFoundError: Could not initialize class org.jboss.classfilewriter.ClassFile
at org.jboss.weld.bean.proxy.ProxyFactory.newClassFile(ProxyFactory.java:494)
at org.jboss.weld.bean.proxy.ProxyFactory.createProxyClass(ProxyFactory.java:444)
at org.jboss.weld.bean.proxy.ProxyFactory.getProxyClass(ProxyFactory.java:370)