stevenschlansker opened a new issue, #11760:
URL: https://github.com/apache/maven/issues/11760
### Affected version
4.0.0-rc-5
### Bug description
I am attempting to upgrade a project to Maven 4. The project includes a
build parent pom, which is a sibling module to others that use the parent.
```
jdbi/
pom.xml
jdbi-build-parent/
pom.xml
jdbi-core/
pom.xml (inherit build-parent)
src/...
jdbi-other-integration/...
```
The build parent pom itself inherits from a generic template pom (
[basepom](https://basepom.org/) )
In order to combine e.g. compiler arguments in our build parent and basepom,
in our build parent we declare:
```xml
<project xmlns="http://maven.apache.org/POM/4.0.0";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xmlns:mvn="http://maven.apache.org/POM/4.0.0";
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
https://maven.apache.org/xsd/maven-4.0.0.xsd";>
...
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<executions>
<execution>
<id>default-compile</id>
<configuration>
<compilerArgs mvn:combine.children="append">
<arg>-Xlint:deprecation</arg>
<arg>-Xlint:unchecked</arg>
</compilerArgs>
</configuration>
</execution>
</plugin>
...
```
We added the `mvn` namespace since it seemed to be the upgrade path from the
old `combine.children` directive.
When building the multi-module project, all is well.
```
% mvn clean install
...
[INFO] BUILD SUCCESS
```
However, if you then try to use an individual project alone, without the
full reactor build, you get a surprising error:
```
% mvn test -pl :jdbi-other-integration
...
[WARNING] The POM for org.jdbi:jdbi-core:jar:tests:4.0.0-SNAPSHOT is
invalid, transitive dependencies (if any) will not be available: 1 problem was
- [FATAL] Non-parseable POM org.jdbi:jdbi-build-parent:4.0.0-SNAPSHOT:
Unable to read model: Undeclared namespace prefix "mvn" (for attribute
"combine.children")
at [row,col {unknown-source}]: [786,64]
...
[ERROR]
org.jdbi.guava.codec.TypeResolvingCodecFactoryTest.testCollectionCodec -- Time
elapsed: 0.101 s <<< ERROR!
java.lang.NoClassDefFoundError: io/leangen/geantyref/GenericTypeReflector
```
Sure enough, the installed consumer pom lost the `xmlns:mvn` on the opening
`<project>` tag:
```
% head
~/.m2/repository/org/jdbi/jdbi-build-parent/4.0.0-SNAPSHOT/jdbi-build-parent-4.0.0-SNAPSHOT.pom
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
https://maven.apache.org/xsd/maven-4.0.0.xsd";>
```
So it looks like Maven, while transforming our build parent pom into its
consumer version for installation to the local repository, drops the `<project
xmlns:mvn` declaration and then writes an invalid POM file to the local repo.
The source is available, although it's not a minimal example:
https://github.com/jdbi/jdbi/pull/2935
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