Please answer these questions before submitting your issue. Thanks!

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

go version devel +f8b0231639 Mon Mar 20 20:54:19 2017 +0000 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/usr/local/google/home/shadams/go"
GORACE=""
GOROOT="/usr/local/google/home/shadams/dev/go"
GOTOOLDIR="/usr/local/google/home/shadams/dev/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build400923614=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

package main

import (
	"encoding/xml"
	"fmt"
)

type Parent struct {
	I     int
	Child struct{ I int }
}

const emptyXML = `
<Parent>
    <I></I>
    <Child></Child>
</Parent>
`

func main() {
	p := &Parent{I: 1, Child: struct{ I int }{2}}
	if err := xml.Unmarshal([]byte(emptyXML), p); err != nil {
		panic(err)
	}

	fmt.Printf("int (%v) vs struct (%v)\n", p.I, p.Child)
}

What did you expect to see?

int (0) vs struct ({0})

What did you see instead?

int (0) vs struct ({2})

An empty xml val always results in a zero-valued destination, regardless of any pre-populated values, except if type is struct, ptr to struct, slice of struct, etc.

This was existing behavior before https://go-review.googlesource.com/c/38386/. Not sure if it makes sense to change at all, but wanted to document the inconsistency.

cc @rsc @bradfitz @ianlancetaylor