gnodet commented on issue #11760:
URL: https://github.com/apache/maven/issues/11760#issuecomment-4117681086
_Claude Code on behalf of Guillaume Nodet_
After investigating this issue, we found that it actually involves two
separate problems — one is a genuine Maven 4 regression, the other is a
pre-existing misunderstanding of how XML namespaces work with attributes.
## 1. The real Maven 4 bug: invalid consumer POM (now fixed in #11823)
The consumer POM transformation drops the `xmlns:mvn` declaration (which
lives on the `<project>` element and is lost during model serialization), but
the prefixed attributes like `mvn:combine.children` survive in plugin
configuration XML nodes. This produces invalid XML with undeclared namespace
prefixes:
```xml
<!-- consumer POM output — invalid XML -->
<compilerArgs mvn:combine.children="append"> <!-- mvn: prefix is
undeclared! -->
```
This is fixed in #11823 by properly handling namespace declarations and
prefixed attributes on the write side. When a prefixed attribute has no
corresponding `xmlns:` declaration, the prefix is stripped to produce valid XML.
## 2. `mvn:combine.children` never actually worked (not a regression)
This is the more subtle part. The `mvn:combine.children` attribute **never
triggered the append merge behavior**, even in Maven 3. We verified this by
testing with Maven 3.9.11 using a parent-child project:
**Parent POM** defines `<compilerArgs>` with `-Xlint:deprecation`.
**Child with `mvn:combine.children="append"`** (prefixed):
```xml
<compilerArgs mvn:combine.children="append">
<arg>-Xlint:unchecked</arg>
</compilerArgs>
```
Effective POM shows **only** `-Xlint:unchecked` — the parent's
`-Xlint:deprecation` is gone. The append directive was silently ignored.
**Child with `combine.children="append"`** (unprefixed):
```xml
<compilerArgs combine.children="append">
<arg>-Xlint:unchecked</arg>
</compilerArgs>
```
Effective POM shows **both** `-Xlint:deprecation` and `-Xlint:unchecked` —
append works correctly.
### Why?
Per the [XML Namespaces spec (Section
6.2)](https://www.w3.org/TR/xml-names/#defaulting), the default namespace
declaration does **not** apply to attributes. Unprefixed attributes are in **no
namespace** — not the default namespace, not any namespace. So:
- `combine.children` (unprefixed) → in **no namespace**
- `mvn:combine.children` with
`xmlns:mvn="http://maven.apache.org/POM/4.0.0"` → in the **POM namespace**
These are different attributes per the XML spec. Maven's merge logic looks
up `combine.children` (the unprefixed form), so the prefixed form is never
found.
## Recommendation for the jdbi project
The fix is simple: use unprefixed `combine.children` instead of
`mvn:combine.children`, and remove the `xmlns:mvn` declaration:
```xml
<project xmlns="http://maven.apache.org/POM/4.0.0";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="...">
...
<compilerArgs combine.children="append">
<arg>-Xlint:deprecation</arg>
<arg>-Xlint:unchecked</arg>
</compilerArgs>
...
</project>
```
This works in both Maven 3 and Maven 4.
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