golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898: Opaque

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 golang.org/x/xerrors.Opaque in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898
Demonstrate these symbols/APIs:
  - golang.org/x/xerrors.Opaque

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:sha256:55365a84cfa581ac308634cc41ebfa8650468c93092a18db644c307f7d78b7ad","contract":["xerrors.Opaque(wrapped) returns a non-nil error whose Error() string equals the wrapped error message","xerrors.Unwrap(xerrors.Opaque(wrapped)) returns nil","xerrors.Is(xerrors.Opaque(wrapped), wrapped) and xerrors.Is(xerrors.Opaque(wrapped), base) return false","xerrors.As(xerrors.Opaque(wrapped), \u0026target) returns false and does not unwrap into the underlying custom error type"],"goal":"verify golang.org/x/xerrors.Opaque in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","kind":"HOW","packages":["pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"symbols":["golang.org/x/xerrors.Opaque"]},"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/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898","symbols":["golang.org/x/xerrors.Opaque"],"verifierAdapter":"golang@1"}

module example.com/sample

go 1.21

require golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898

golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/xerrors.Opaque in pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/xerrors@v0.0.0-20191011141410-1b5146add898"
  ],
  "symbols": [
    "golang.org/x/xerrors.Opaque"
  ]
}

package main

import (
	"fmt"
	"os"

	"golang.org/x/xerrors"
)

type customErr struct {
	msg string
}

func (e *customErr) Error() string {
	return e.msg
}

func main() {
	base := &customErr{msg: "database timeout"}
	wrapped := xerrors.Errorf("query failed: %w", base)
	if !xerrors.Is(wrapped, base) {
		fmt.Fprintln(os.Stderr, "expected wrapped error to match base before Opaque")
		os.Exit(1)
	}

	opaqueErr := xerrors.Opaque(wrapped)
	if opaqueErr == nil {
		fmt.Fprintln(os.Stderr, "expected non-nil error from Opaque")
		os.Exit(1)
	}
	if opaqueErr.Error() != "query failed: database timeout" {
		fmt.Fprintf(os.Stderr, "unexpected Error() string: %q\n", opaqueErr.Error())
		os.Exit(1)
	}
	if formatted := fmt.Sprintf("%+v", opaqueErr); formatted == "" {
		fmt.Fprintln(os.Stderr, "expected non-empty formatted output")
		os.Exit(1)
	}
	if unwrapped := xerrors.Unwrap(opaqueErr); unwrapped != nil {
		fmt.Fprintf(os.Stderr, "expected Unwrap(opaqueErr) == nil, got %v\n", unwrapped)
		os.Exit(1)
	}
	if xerrors.Is(opaqueErr, wrapped) || xerrors.Is(opaqueErr, base) {
		fmt.Fprintln(os.Stderr, "expected Opaque error not to match wrapped or base via Is")
		os.Exit(1)
	}
	var target *customErr
	if xerrors.As(opaqueErr, &target) {
		fmt.Fprintln(os.Stderr, "expected Opaque error not to unwrap into *customErr via As")
		os.Exit(1)
	}
}