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

$ go version
go version go1.17.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/aron/Library/Caches/go-build"
GOENV="/Users/aron/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/aron/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/aron/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/8j/pt4fjhkj11d3xd9344pdspmh0000gn/T/go-build3937592509=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

We have incoming JSON that receives a Boolean value as a quoted number. We have a custom UnmarshalJSON method that helps parse different types of Boolean values. When that incoming value comes in with an unexpected, empty string value, the unmarshal function is never called; the decoder decides that this situation is an error without giving the target type an opportunity to handle the value.

Perhaps this is an unexpected, unsupported situation? The ,string documentation indicates that it can be used for Boolean types, but we have added a custom UnmarshalJSON to the mix.

This same type has caused us struggles in the past; see #20651

package main

import (
	"encoding/json"
	"fmt"
	"strconv"
)

type parsedBool bool

func (p *parsedBool) UnmarshalJSON(data []byte) error {
	v, err := strconv.ParseBool(string(data))
	if err != nil {
		return fmt.Errorf("custom error: %w", err)
	}
	*p = parsedBool(v)
	return nil
}

type Target struct {
	Enabled parsedBool `json:"enabled,string"`
}

func main() {
	var err error
        empty := `{"enabled":""}`
        truth := `{"enabled":"t"}`
        unexpected := `{"enabled":"unexpected"}`

	target := Target{}
	err = json.Unmarshal([]byte(empty), &target)
	fmt.Printf("target: %#v; error: %s\n", target, err)
	err = json.Unmarshal([]byte(unexpected), &target)
	fmt.Printf("target: %#v; error: %s\n", target, err)
	err = json.Unmarshal([]byte(truth), &target)
	fmt.Printf("target: %#v; error: %s\n", target, err)
	
}

https://play.golang.com/p/bFAQ4Xu6mar

What did you expect to see?

Expected to see parsedBool#UnmarshalJSON attempt parse the incoming value, producing something like:

target: main.Target{Enabled:false}; error: custom error: strconv.ParseBool: parsing "": invalid syntax
target: main.Target{Enabled:false}; error: custom error: strconv.ParseBool: parsing "unexpected": invalid syntax
target: main.Target{Enabled:true}; error: %!s(<nil>)

What did you see instead?

An error about an invalid ,string struct tag, which prevents the call to UnmarshalJSON.

target: main.Target{Enabled:false}; error: json: invalid use of ,string struct tag, trying to unmarshal "" into main.parsedBool
target: main.Target{Enabled:false}; error: custom error: strconv.ParseBool: parsing "unexpected": invalid syntax
target: main.Target{Enabled:true}; error: %!s(<nil>)