Bug: Case-insensitive key matching fails for structs with 17+ fields

[arkus7]

Summary

goccy/go-json fails to perform case-insensitive JSON key matching (as specified by encoding/json) when the target struct has 17 or more visible JSON fields (fields with a json tag that is not "-"). Structs with 16 or fewer fields work correctly.

Versions Affected

All tested versions are affected:

• v0.9.7
• v0.10.5

Expected Behavior

Per the encoding/json specification:

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match.

goccy/go-json documents itself as a drop-in replacement for encoding/json. Case-insensitive matching should work regardless of the number of fields in the struct.

Reproduction

package main

import (
	"encoding/json"
	"fmt"

	goccy "github.com/goccy/go-json"
)

// 16 fields — case-insensitive matching works
type small struct {
	F1   string `json:"f1"`
	F2   string `json:"f2"`
	F3   string `json:"f3"`
	F4   string `json:"f4"`
	F5   string `json:"f5"`
	F6   string `json:"f6"`
	F7   string `json:"f7"`
	F8   string `json:"f8"`
	F9   string `json:"f9"`
	F10  string `json:"f10"`
	F11  string `json:"f11"`
	F12  string `json:"f12"`
	F13  string `json:"f13"`
	F14  string `json:"f14"`
	F15  string `json:"f15"`
	Name bool   `json:"name,omitempty"`
}

// 17 fields — case-insensitive matching FAILS
type large struct {
	F1   string `json:"f1"`
	F2   string `json:"f2"`
	F3   string `json:"f3"`
	F4   string `json:"f4"`
	F5   string `json:"f5"`
	F6   string `json:"f6"`
	F7   string `json:"f7"`
	F8   string `json:"f8"`
	F9   string `json:"f9"`
	F10  string `json:"f10"`
	F11  string `json:"f11"`
	F12  string `json:"f12"`
	F13  string `json:"f13"`
	F14  string `json:"f14"`
	F15  string `json:"f15"`
	F16  string `json:"f16"`
	Name bool   `json:"name,omitempty"`
}

func main() {
	// JSON key "Name" should match struct tag "name" (case-insensitive)
	input := []byte(`{"Name":true}`)
	fmt.Printf("Input: %s\n\n", input)

	// 16 fields - works
	var g1 small
	goccy.Unmarshal(input, &g1)
	var s1 small
	json.Unmarshal(input, &s1)
	fmt.Printf("[16 fields] goccy: Name=%v | encoding/json: Name=%v\n", g1.Name, s1.Name)

	// 17 fields - BUG: goccy fails to match
	var g2 large
	goccy.Unmarshal(input, &g2)
	var s2 large
	json.Unmarshal(input, &s2)
	fmt.Printf("[17 fields] goccy: Name=%v | encoding/json: Name=%v\n", g2.Name, s2.Name)
}

Output

Input: {"Name":true}

[16 fields] goccy: Name=true | encoding/json: Name=true
[17 fields] goccy: Name=false | encoding/json: Name=true

Test with ignored field - works when visible fields number is <= 16

Test case with 17 fields when one of them is ignored (`json:"-"`)

package main

import (
	"encoding/json"
	"fmt"

	goccy "github.com/goccy/go-json"
)

// 16 fields — case-insensitive matching works
type small struct {
	F1   string `json:"f1"`
	F2   string `json:"f2"`
	F3   string `json:"f3"`
	F4   string `json:"f4"`
	F5   string `json:"f5"`
	F6   string `json:"f6"`
	F7   string `json:"f7"`
	F8   string `json:"f8"`
	F9   string `json:"f9"`
	F10  string `json:"f10"`
	F11  string `json:"f11"`
	F12  string `json:"f12"`
	F13  string `json:"f13"`
	F14  string `json:"f14"`
	F15  string `json:"f15"`
	Name bool   `json:"name,omitempty"`
}

// 17 fields — case-insensitive matching FAILS
type large struct {
	F1   string `json:"f1"`
	F2   string `json:"f2"`
	F3   string `json:"f3"`
	F4   string `json:"f4"`
	F5   string `json:"f5"`
	F6   string `json:"f6"`
	F7   string `json:"f7"`
	F8   string `json:"f8"`
	F9   string `json:"f9"`
	F10  string `json:"f10"`
	F11  string `json:"f11"`
	F12  string `json:"f12"`
	F13  string `json:"f13"`
	F14  string `json:"f14"`
	F15  string `json:"f15"`
	F16  string `json:"-"`
	Name bool   `json:"name,omitempty"`
}

func main() {
	// JSON key "Name" should match struct tag "name" (case-insensitive)
	input := []byte(`{"Name":true}`)
	fmt.Printf("Input: %s\n\n", input)

	// 16 fields - works
	var g1 small
	goccy.Unmarshal(input, &g1)
	var s1 small
	json.Unmarshal(input, &s1)
	fmt.Printf("[16 fields] goccy: Name=%v | encoding/json: Name=%v\n", g1.Name, s1.Name)

	// 17 fields - works because of ignored field
	var g2 large
	goccy.Unmarshal(input, &g2)
	var s2 large
	json.Unmarshal(input, &s2)
	fmt.Printf("[17 fields] goccy: Name=%v | encoding/json: Name=%v\n", g2.Name, s2.Name)
}

Output

Input: {"Name":true}

[16 fields] goccy: Name=true | encoding/json: Name=true
[17 fields] goccy: Name=true | encoding/json: Name=true

Impact

This is a silent data loss bug - no error is returned, the field is simply left at its zero value. This makes it particularly dangerous because:

1. It only manifests when a struct grows past 16 fields, which can happen during normal development
2. It only affects requests that rely on case-insensitive matching, so it may not be caught by tests that use exact-case keys
3. There is no error to catch - Unmarshal returns nil

I've discovered this in production when customers' API requests started failing after we added new fields to a struct that previously had 16 visible JSON fields.