encoding/xml: Marshalling change for namespace of XML-element without xml tag in corresponding go-struct in go 1.21+

[LenaBullens]

What version of Go are you using (go version)?

1.21.4

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

windows/amd64

What did you do?

Consider following code snippet:

func main() {
	type B struct {
		XMLName xml.Name
		C       bool     `xml:"c"`
	}

	type A struct {
		XMLName xml.Name `xml:"http://example.com a"`
		B       B
	}

	a := A{
		B: B{
			C: true,
		},
	}

	output, err := xml.Marshal(&a)
	if err != nil {
		fmt.Println(err)
	}

	fmt.Printf("output: %s", string(output))
}

Output in go 1.20.X

output: <a xmlns="http://example.com"><B><c>true</c></B></a>

Output in go 1.21+ (including 1.21.4)

output: <a xmlns="http://example.com"><B xmlns=""><c>true</c></B></a>

Remarks

This issue seems related to #61881 If one adds a tag to the XMLName field in B

	type B struct {
		XMLName xml.Name `xml:"b"`
		C       bool     `xml:"c"`
	}

then the empty namespace does not show up in the marshall result (in 1.21.1+).

[seankhliao]

I believe this is working as intended.
The order of precedence we have is:

• the tag on the XMLName field, if the data is a struct
• the value of the XMLName field of type Name
• the tag of the struct field used to obtain the data
• the name of the struct field used to obtain the data
• the name of the marshaled type

and #7113 / CL 108796 requires the enforcement of xmlns values from xml.Name