github.com/imdario/mergo v0.3.12: ErrNonPointerAgument

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.ErrNonPointerAgument in pkg:golang/github.com/imdario/mergo@v0.3.12
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/imdario/mergo@v0.3.12
Demonstrate these symbols/APIs:
  - github.com/imdario/mergo.ErrNonPointerAgument

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:sha256:4feba3f9b4d86fd50b283f51aa4e630c46412ac35f38f472c532a07b55922a86","contract":["mergo.ErrNonPointerAgument has error message 'dst must be a pointer' and satisfies errors.Is comparison","mergo.Merge returns ErrNonPointerAgument when destination is a non-pointer struct value","mergo.Merge returns ErrNonPointerAgument when destination is a non-pointer map value","mergo.Map returns ErrNonPointerAgument when destination is a non-pointer map","mergo.Merge succeeds without ErrNonPointerAgument when destination is a pointer to a struct or map"],"goal":"verify github.com/imdario/mergo.ErrNonPointerAgument in pkg:golang/github.com/imdario/mergo@v0.3.12","kind":"HOW","packages":["pkg:golang/github.com/imdario/mergo@v0.3.12"],"schemaVersion":1,"symbols":["github.com/imdario/mergo.ErrNonPointerAgument"]},"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@v0.3.12"],"schemaVersion":1,"subject":"pkg:golang/github.com/imdario/mergo@v0.3.12","symbols":["github.com/imdario/mergo.ErrNonPointerAgument"],"verifierAdapter":"golang@1"}

module sample

go 1.26.6

require github.com/imdario/mergo v0.3.12

github.com/imdario/mergo v0.3.12 h1:b6R2BslTbIEToALKP7LxUvijTsNI9TAe80pLWN2g/HU=
github.com/imdario/mergo v0.3.12/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

{
  "schemaVersion": 1,
  "goal": "verify github.com/imdario/mergo.ErrNonPointerAgument in pkg:golang/github.com/imdario/mergo@v0.3.12",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/imdario/mergo@v0.3.12"
  ],
  "symbols": [
    "github.com/imdario/mergo.ErrNonPointerAgument"
  ]
}

package main

import (
	"errors"
	"fmt"
	"os"

	"github.com/imdario/mergo"
)

type Config struct {
	Host string
	Port int
}

func main() {
	if err := runContractTests(); err != nil {
		fmt.Fprintf(os.Stderr, "Contract test failed: %v\n", err)
		os.Exit(1)
	}
	fmt.Println("All contract tests passed successfully.")
}

func runContractTests() error {
	// Contract 1: mergo.ErrNonPointerAgument has error message "dst must be a pointer" and satisfies errors.Is comparison
	if got := mergo.ErrNonPointerAgument.Error(); got != "dst must be a pointer" {
		return fmt.Errorf("mergo.ErrNonPointerAgument error string mismatch: got %q, want %q", got, "dst must be a pointer")
	}

	// Contract 2: mergo.ErrNonPointerAgument is returned by mergo.Merge when destination is a non-pointer struct value
	dstStruct := Config{Port: 8080}
	srcStruct := Config{Host: "localhost", Port: 9000}
	errStruct := mergo.Merge(dstStruct, srcStruct)
	if errStruct == nil {
		return fmt.Errorf("mergo.Merge with non-pointer struct expected error, got nil")
	}
	if !errors.Is(errStruct, mergo.ErrNonPointerAgument) {
		return fmt.Errorf("mergo.Merge with non-pointer struct returned %v, want mergo.ErrNonPointerAgument", errStruct)
	}

	// Contract 3: mergo.ErrNonPointerAgument is returned by mergo.Merge when destination is a non-pointer map value
	dstMap := map[string]string{"env": "prod"}
	srcMap := map[string]string{"host": "localhost", "env": "dev"}
	errMap := mergo.Merge(dstMap, srcMap)
	if errMap == nil {
		return fmt.Errorf("mergo.Merge with non-pointer map expected error, got nil")
	}
	if !errors.Is(errMap, mergo.ErrNonPointerAgument) {
		return fmt.Errorf("mergo.Merge with non-pointer map returned %v, want mergo.ErrNonPointerAgument", errMap)
	}

	// Contract 4: mergo.ErrNonPointerAgument is returned by mergo.Map when destination is a non-pointer map
	dstMapForMap := map[string]interface{}{"port": 8080}
	srcMapForMap := map[string]interface{}{"host": "localhost"}
	errMapCall := mergo.Map(dstMapForMap, srcMapForMap)
	if errMapCall == nil {
		return fmt.Errorf("mergo.Map with non-pointer map expected error, got nil")
	}
	if !errors.Is(errMapCall, mergo.ErrNonPointerAgument) {
		return fmt.Errorf("mergo.Map with non-pointer map returned %v, want mergo.ErrNonPointerAgument", errMapCall)
	}

	// Contract 5: mergo.Merge succeeds without ErrNonPointerAgument when destination is a pointer to a struct or map
	dstPtrStruct := &Config{Port: 8080}
	if err := mergo.Merge(dstPtrStruct, srcStruct); err != nil {
		return fmt.Errorf("mergo.Merge with pointer struct failed: %w", err)
	}
	if dstPtrStruct.Host != "localhost" || dstPtrStruct.Port != 8080 {
		return fmt.Errorf("mergo.Merge with pointer struct unexpected state: %+v", dstPtrStruct)
	}

	dstPtrMap := &map[string]string{"env": "prod"}
	if err := mergo.Merge(dstPtrMap, srcMap); err != nil {
		return fmt.Errorf("mergo.Merge with pointer map failed: %w", err)
	}
	if (*dstPtrMap)["host"] != "localhost" || (*dstPtrMap)["env"] != "prod" {
		return fmt.Errorf("mergo.Merge with pointer map unexpected state: %+v", *dstPtrMap)
	}

	return nil
}