go: cmd/vet: Consider reverting tag conflict for embedded fields

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

$ go version
go version go1.12 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/pschultz/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/pschultz/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build788821326=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run go vet on existing code base, containing code such as this: https://play.golang.org/p/HnkPcNUu84J

What did you expect to see?

go vet to either succeed, or to report only real problems.

What did you see instead?

go vet fails due to intended shadowing of embedded fields with json tags.

#25593 changed vet to also report repeated tags from embedded types.

We use this technique intentionally to create slightly varying JSON representation for struct types, typically when returning large lists of a particular type. Here is a simplified example (playground):

package main

import (
    "encoding/json"
    "fmt"
)

type Book struct {
    Author Author `json:"author"`
    Title  string `json:"title"`
}

type Author struct {
    Name      string `json:"name"`
    Biography string `json:"biography"`
}

func main() {
    b := Book{
        Title: "Foo",
        Author: Author{
            Name:      "John Doe",
            Biography: "Lorem Ipsum",
        },
    }

    // When returning a list of books, we don't want to include the whole
    // Author struct for each book.
    type BookSummary struct {
        Book            // Include all Book fields by default,
        Author struct { // but override the "author" field with a smaller model
            Name string `json:"name"`
            // All other fields omitted
        } `json:"author"`
    }

    var s BookSummary
    s.Book = b
    s.Author.Name = b.Author.Name

    j, _ := json.MarshalIndent(s, "", "  ")
    fmt.Println(string(j))
}

With go vet now rejecting this code, the only viable alternative I can see is to redefine the Book type completely:

type BookSummary struct {
    Title  string `json:"title"`
    Author struct {
        Name string `json:"name"`
    } `json:"author"`
    // Many more fields
}

This way, however, the JSON representation of BookSummary will not be updated automatically when Book fields are added or changed. We would also have to assign each field individually, since Book and BookSummary don’t have the same underlying type. Realistically, this forces us to do some kind of code generation, which is clearly much more trouble than what worked with Go 1.11.

Other possible fixes are:

• Not run go vet anymore
• Marshal the Book value, then unmarshal into a map, then modify the map, and then marshal again.
• Add the MarshalJSON method and some fields that control how that method behaves.

None of these are appealing.

Please consider this as a valid use-case for repeated json tags on embedded fields and revert 4b439e41e2cdd78e0eeed05942c93364c5d99b6b.