github.com/imdario/mergo v0.3.8: WithOverride

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify github.com/imdario/mergo.WithOverride in pkg:golang/github.com/imdario/mergo@v0.3.8
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/imdario/mergo@v0.3.8
Demonstrate these symbols/APIs:
  - github.com/imdario/mergo.WithOverride

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:79930199c1d807661ff9e4a0ce47b1bd3544a8482bab4337136055256d849ce6","contract":["mergo.Merge with WithOverride overwrites non-empty destination struct fields with non-empty source values","mergo.Merge without WithOverride preserves existing non-empty destination struct fields","mergo.Merge with WithOverride does not overwrite non-empty destination fields with zero-value source fields","mergo.Merge with WithOverride overrides matching keys in destination map with source map values"],"goal":"verify github.com/imdario/mergo.WithOverride in pkg:golang/github.com/imdario/mergo@v0.3.8","kind":"HOW","packages":["pkg:golang/github.com/imdario/mergo@v0.3.8"],"schemaVersion":1,"symbols":["github.com/imdario/mergo.WithOverride"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/imdario/mergo@v0.3.8"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.8","symbols":["github.com/imdario/mergo.WithOverride"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.26.6

require github.com/imdario/mergo v0.3.8

github.com/imdario/mergo v0.3.8 h1:CGgOkSJeqMRmt0D9XLWExdT4m4F1vd3FV3VPt+0VxkQ=
github.com/imdario/mergo v0.3.8/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=

package main

import (
	"fmt"
	"log"

	"github.com/imdario/mergo"
)

// ServerConfig holds configuration settings.
type ServerConfig struct {
	Host    string
	Port    int
	Timeout int
	Debug   bool
}

// MergeConfig merges source configuration into destination.
// When override is true, WithOverride is used to overwrite non-empty destination fields.
func MergeConfig(dst *ServerConfig, src ServerConfig, override bool) error {
	var opts []func(*mergo.Config)
	if override {
		opts = append(opts, mergo.WithOverride)
	}
	return mergo.Merge(dst, src, opts...)
}

func main() {
	dst := ServerConfig{
		Host:    "localhost",
		Port:    8080,
		Timeout: 30,
		Debug:   false,
	}

	src := ServerConfig{
		Port:    9090,
		Timeout: 60,
	}

	if err := MergeConfig(&dst, src, true); err != nil {
		log.Fatalf("failed to merge configuration: %v", err)
	}

	fmt.Printf("Merged Config: Host=%s Port=%d Timeout=%d Debug=%t\n",
		dst.Host, dst.Port, dst.Timeout, dst.Debug)
}

{
  "schemaVersion": 1,
  "goal": "verify github.com/imdario/mergo.WithOverride in pkg:golang/github.com/imdario/mergo@v0.3.8",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/imdario/mergo@v0.3.8"
  ],
  "symbols": [
    "github.com/imdario/mergo.WithOverride"
  ]
}

package main

import (
	"fmt"
	"os"

	"github.com/imdario/mergo"
)

type Config struct {
	Host    string
	Port    int
	Timeout int
}

func main() {
	// Assertion 1: mergo.Merge with WithOverride overwrites non-empty destination struct fields with non-empty source values
	dst1 := Config{Host: "localhost", Port: 8080, Timeout: 30}
	src1 := Config{Port: 9090, Timeout: 60}
	if err := mergo.Merge(&dst1, src1, mergo.WithOverride); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 1 failed with error: %v\n", err)
		os.Exit(1)
	}
	if dst1.Host != "localhost" || dst1.Port != 9090 || dst1.Timeout != 60 {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 1 expected Host=localhost, Port=9090, Timeout=60; got %+v\n", dst1)
		os.Exit(1)
	}

	// Assertion 2: mergo.Merge without WithOverride preserves existing non-empty destination struct fields
	dst2 := Config{Host: "localhost", Port: 8080}
	src2 := Config{Port: 9090, Timeout: 60}
	if err := mergo.Merge(&dst2, src2); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 failed with error: %v\n", err)
		os.Exit(1)
	}
	if dst2.Host != "localhost" || dst2.Port != 8080 || dst2.Timeout != 60 {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 2 expected Host=localhost, Port=8080, Timeout=60; got %+v\n", dst2)
		os.Exit(1)
	}

	// Assertion 3: mergo.Merge with WithOverride does not overwrite non-empty destination fields with zero-value source fields
	dst3 := Config{Host: "localhost", Port: 8080, Timeout: 30}
	src3 := Config{Host: "", Port: 9090}
	if err := mergo.Merge(&dst3, src3, mergo.WithOverride); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 failed with error: %v\n", err)
		os.Exit(1)
	}
	if dst3.Host != "localhost" || dst3.Port != 9090 || dst3.Timeout != 30 {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 3 expected Host=localhost, Port=9090, Timeout=30; got %+v\n", dst3)
		os.Exit(1)
	}

	// Assertion 4: mergo.Merge with WithOverride overrides matching keys in destination map with source map values
	dstMap := map[string]int{"a": 1, "b": 2}
	srcMap := map[string]int{"b": 20, "c": 30}
	if err := mergo.Merge(&dstMap, srcMap, mergo.WithOverride); err != nil {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 4 failed with error: %v\n", err)
		os.Exit(1)
	}
	if dstMap["a"] != 1 || dstMap["b"] != 20 || dstMap["c"] != 30 {
		fmt.Fprintf(os.Stderr, "FAIL: assertion 4 expected a=1, b=20, c=30; got %+v\n", dstMap)
		os.Exit(1)
	}

	fmt.Println("All contract assertions passed successfully.")
}