encoding/json: wrong type name used in UnmarshalTypeError

[j2gg0s]

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Go version

go version go1.22.4 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/j2gg0s/Library/Caches/go-build'
GOENV='/Users/j2gg0s/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/j2gg0s/go/pkg/mod'
GONOPROXY='dev.msh.team,ohai.bot,github.com/j2gg0s'
GONOSUMDB='dev.msh.team,ohai.bot,github.com/j2gg0s'
GOOS='darwin'
GOPATH='/Users/j2gg0s/go'
GOPRIVATE='dev.msh.team,ohai.bot,github.com/j2gg0s'
GOPROXY='https://goproxy.cn,direct'
GOROOT='/Users/j2gg0s/sdk/go1.22.4'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='go1.22.4'
GOTOOLDIR='/Users/j2gg0s/sdk/go1.22.4/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.4'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/j2gg0s/gosrc/src/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/3g/291pfdd54f11rlb15_hbq1dr0000gn/T/go-build1558397162=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

package main

import (
	"encoding/json"
	"fmt"
)

type Y struct {
	ZField int
}

type X struct {
	YField Y
}

func ExampleUnmarshalTypeError() {
	x := X{}
	if err := json.Unmarshal([]byte(`{"YField": {"ZField": "hello"}}`), &x); err != nil {
		fmt.Println(err)
	}
	// Output:
	// json: cannot unmarshal string into Go struct field Y.YField.ZField of type int
}

What did you see happen?

Error's message is json: cannot unmarshal string into Go struct field Y.YField.ZField of type int

What did you expect to see?

Error's message should bejson: cannot unmarshal string into Go struct field X.YField.ZField of type int

If my understanding is correct, can I submit a PR to fix this issue?

[gabyhelp]

Related Issues and Documentation

• encoding/json: nested unmarshaling mangles errors #68750 (closed)
• encoding/json: UnmarshalTypeError is clobbered when using nested custom marshallers #61337 (closed)
• encoding/json: UnmarshalJSON not working consistently with struct embedding #39175 (closed)
• encoding/json: UnmarshalTypeError Offset is broken if a nested struct implements UnmarshalJSON #43686 (closed)
• encoding/json: Decoding returns error when case-insensitive keys overlap and not all struct fields defined #54404 (closed)
• encoding/json: unmarshaler doesn't find UnmarshalJSON on embedded anonymous struct #46516
• encoding/json does not unmarshal my struct properly #63161 (closed)
• One more point #42519 (closed)
• encoding/json: promoted Unmarshal method on embedded field caused confusion #39470 (closed)
• encoding/json: json encoder fails for embedded non-struct fields #4474 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[j2gg0s]

Additional information about the solution.

Can we insert the Struct as the first element of the Field and no longer maintain the Struct field. decode.go#L123

// An UnmarshalTypeError describes a JSON value that was
// not appropriate for a value of a specific Go type.
type UnmarshalTypeError struct {
	Value  string       // description of JSON value - "bool", "array", "number -5"
	Type   reflect.Type // type of Go value it could not be assigned to
	Offset int64        // error occurred after reading Offset bytes
	Struct string       // name of the struct type containing the field
	Field  string       // the full path from root node to the field
}

[gopherbot]

Change https://go.dev/cl/606956 mentions this issue: encoding/json: print correct field path in UnmarshalTypeError

[j2gg0s]

At the same time, we also lose the embedded struct information.

package main

import (
	"encoding/json"
	"fmt"
)

type Y struct {
	ZField int
}

type X struct {
	YField Y
}

type A struct {
	IntField int
}
type B struct {
	A
}
type C struct {
	B
}

func ExampleUnmarshalTypeError() {
	x := X{}
	if err := json.Unmarshal([]byte(`{"YField": {"ZField": "hello"}}`), &x); err != nil {
		fmt.Println(err)
	}
	c := C{}
	if err := json.Unmarshal([]byte(`{"IntField": "hello"}`), &c); err != nil {
		fmt.Println(err)
	}
	// Output:
	// json: cannot unmarshal string into Go struct field Y.YField.ZField of type int
	// json: cannot unmarshal string into Go struct field C.IntField of type int
}