main.go

package main

import (
	"encoding/xml"
	"fmt"
	"time"
)

var dataSingle = `
<response>
  <info>
    <lastmodified>2024-01-14T10:53:21Z</lastmodified>
  </info>
  <a><id>1</id></a>
</response>
`

var dataMultiple = `
<response>
  <info>
    <lastmodified>2024-01-14T10:53:21Z</lastmodified>
  </info>
  <a><id>1</id></a>
  <a><id>2</id></a>
</response>
`

type A1 struct {
	ID string `xml:"id"`
}

type Response1 struct {
	XMLName      xml.Name   `xml:"response"`
	A            []A1       `xml:"a"`
	LastModified *time.Time `xml:"info>lastmodified"`
}

type Response2 struct {
	XMLName      xml.Name   `xml:"response"`
	Data         []A1       `xml:"a"`
	LastModified *time.Time `xml:"info>lastmodified"`
}

type A3 struct {
	XMLName xml.Name `xml:"a"`
	ID      string   `xml:"id"`
}

type Response3 struct {
	XMLName      xml.Name `xml:"response"`
	Data         []A3
	LastModified *time.Time `xml:"info>lastmodified"`
}

type Response4[T Responses4] struct {
	XMLName      xml.Name `xml:"response"`
	Data         []T
	LastModified *time.Time `xml:"info>lastmodified"`
}

type Responses4 interface {
	A4 | RespB
}

type (
	A4 struct {
		XMLName xml.Name `xml:"a"`
		ID      string   `xml:"id"`
	}
	RespB struct{}
)

type Response5[T Responses4] struct {
	XMLName      xml.Name   `xml:"response"`
	Data         []T        `xml:",any"`
	LastModified *time.Time `xml:"info>lastmodified"`
}

func main() {
	fmt.Println("example 1")
	decodeAndPrint(dataSingle, &Response1{})
	decodeAndPrint(dataMultiple, &Response1{})
	fmt.Println("")
	fmt.Println("example 2")
	decodeAndPrint(dataSingle, &Response2{})
	decodeAndPrint(dataMultiple, &Response2{})
	fmt.Println("")
	fmt.Println("example 3")
	decodeAndPrint(dataSingle, &Response3{})
	decodeAndPrint(dataMultiple, &Response3{})
	fmt.Println("")
	fmt.Println("example with generics")
	decodeAndPrint(dataSingle, &Response4[A4]{})
	decodeAndPrint(dataMultiple, &Response4[A4]{})
	fmt.Println("")
	fmt.Println("example with generics and xml any tag")
	decodeAndPrint(dataSingle, &Response5[A4]{})
	decodeAndPrint(dataMultiple, &Response5[A4]{})
}

func decodeAndPrint(data string, res any) {
	if err := xml.Unmarshal([]byte(data), res); err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("%+v\n", res)
}

output

example 1
&{XMLName:{Space: Local:response} A:[{ID:1}] LastModified:2024-01-14 10:53:21 +0000 UTC}
&{XMLName:{Space: Local:response} A:[{ID:1} {ID:2}] LastModified:2024-01-14 10:53:21 +0000 UTC}

example 2
&{XMLName:{Space: Local:response} Data:[{ID:1}] LastModified:2024-01-14 10:53:21 +0000 UTC}
&{XMLName:{Space: Local:response} Data:[{ID:1} {ID:2}] LastModified:2024-01-14 10:53:21 +0000 UTC}

example 3
&{XMLName:{Space: Local:response} Data:[] LastModified:2024-01-14 10:53:21 +0000 UTC}
&{XMLName:{Space: Local:response} Data:[] LastModified:2024-01-14 10:53:21 +0000 UTC}

example with generics
&{XMLName:{Space: Local:response} Data:[] LastModified:2024-01-14 10:53:21 +0000 UTC}
&{XMLName:{Space: Local:response} Data:[] LastModified:2024-01-14 10:53:21 +0000 UTC}

example with generics and xml any tag
&{XMLName:{Space: Local:response} Data:[{XMLName:{Space: Local:a} ID:1}] LastModified:2024-01-14 10:53:21 +0000 UTC}
&{XMLName:{Space: Local:response} Data:[{XMLName:{Space: Local:a} ID:1} {XMLName:{Space: Local:a} ID:2}] LastModified:2024-01-14 10:53:21 +0000 UTC}