XmlRoot and XmlNamespace conflict

[proggeler]

Q	A
Bug report?	yes
Feature request?	yes
BC Break report?	no
RFC?	yes

Both XMLNamespace and XMLRoot can define a namespace and prefix, but it is unclear what takes precedence. They even can conflict when they both point to the same prefix.

It would be better to replace the XmlRoot::prefix with XmlRoot::withPrefix (or usePrefix):

/**
 * @JMS\XmlRoot(name="Foo", namespace="urn.test.ns", withPrefix=true)
 * @JMS\XmlNamespace(uri="urn.test.ns", prefix="ns2")
 */
class Foo
{}

This way the prefix can only be set by the XmlNamespace.

Steps required to reproduce the problem

/**
 * @JMS\XmlRoot(name="Foo", namespace="urn.test.ns", prefix="ns1")
 * @JMS\XmlNamespace(uri="urn.test.ns", prefix="ns2")
 */
class Foo
{
    /**
     * @JMS\Type("int")
     * @JMS\XmlElement(namespace="urn.test.ns")
     */
    public $count = 1;
}

Expected Result

Both point to the same namespace so XmlNamespace takes precedence. (consistent with XmlRoot without prefix)

<?xml version="1.0" encoding="UTF-8"?>
<ns2:Foo xmlns:ns2="urn.test.ns">
  <ns2:count>1</ns2:count>
</ns2:Foo>

Actual Result

XmlRoot prefix is used and there is an useless namespace definition.

<?xml version="1.0" encoding="UTF-8"?>
<ns1:Foo xmlns:ns1="urn.test.ns" xmlns:ns2="urn.test.ns">
  <ns1:count>1</ns1:count>
</ns1:Foo>

Second example

/**
 * @JMS\XmlRoot(name="Foo", namespace="urn.test.ns", prefix="ns1")
 * @JMS\XmlNamespace(uri="urn.test.other.ns", prefix="ns1")
 */
class Foo
{
    /**
     * @JMS\Type("int")
     * @JMS\XmlElement(namespace="urn.test.other.ns")
     */
    public $count = 1;
}

Expected Result

An exception about the conflicting namespaces, or a generated prefix for the prefix that is the second one pointing to the same prefix:

<?xml version="1.0" encoding="UTF-8"?>
<ns-86f87953:Foo xmlns:ns86f87953="urn.test.ns" xmlns:ns1="urn.test.other.ns">
  <ns1:count>1</ns1:count>
</ns1:Foo>

Actual Result

Apparently the XmlNamespace takes precedence now.
This is not valid due to the overriding prefix. (Foo uses the wrong namespace)

<ns1:Foo xmlns:ns1="urn.test.other.ns">
  <ns1:count>1</ns1:count>
</ns1:Foo>