[SUREFIRE-1501] Optional dependencies not correctly handled with JPMS modules

**[Stephen Colebourne](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=scolebourne@joda.org)** opened **[SUREFIRE-1501](https://issues.apache.org/jira/browse/SUREFIRE-1501?redirect=false)** and commented

Attached is a pointlessly simple project for Java 9. The module-info declares an _optional_ dependency on Google Guava (an automatic module) using _requires static_. This optionality is reflected in the pom.xml, where the dependency is marked as optional.

It fails in surefire with JAVA_HOME=jdk9. The reason seems to be that although the Guava jar file is added to the modulepath, it is not referenced using --add-modules. As such, the module graph builder never pulls it in, despite it being on the modulepath. (Optional dependencies need to be directly added using --add-modules if in Maven's dependency graph)

Adding the --add-modules to the `argLine` parameter solves the problem, but this is a bug because the surefire plugin should handle the situation without manually adding it.

NOTE! The attachment has the --add-modules manually added to the pom.xml. Remove it to see the bug.

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**Affects:** 2.21.0

**Attachments:**
- [maven-issue1.zip](https://issues.apache.org/jira/secure/attachment/12914716/maven-issue1.zip) (_4.03 kB_)

1 votes, 4 watchers

Start with the attached maven-issue1.zip and its pom.xml, removing the manually added --add-modules from argLine. Reproduce under JDK 9 and trace how Surefire constructs the module path and module arguments; done means the optional Guava module runs without manual --add-modules.

Written by the indexing model from the issue text.