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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package deepequalerrors defines an Analyzer that checks for the use
// of reflect.DeepEqual with error values.
package deepequalerrors
import (
const Doc = `check for calls of reflect.DeepEqual on error values
The deepequalerrors checker looks for calls of the form:
reflect.DeepEqual(err1, err2)
where err1 and err2 are errors. Using reflect.DeepEqual to compare
errors is discouraged.`
var Analyzer = &analysis.Analyzer{
Name: "deepequalerrors",
Doc: Doc,
URL: "",
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
func run(pass *analysis.Pass) (interface{}, error) {
if !analysisutil.Imports(pass.Pkg, "reflect") {
return nil, nil // doesn't directly import reflect
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
inspect.Preorder(nodeFilter, func(n ast.Node) {
call := n.(*ast.CallExpr)
fn, ok := typeutil.Callee(pass.TypesInfo, call).(*types.Func)
if !ok {
if fn.FullName() == "reflect.DeepEqual" && hasError(pass, call.Args[0]) && hasError(pass, call.Args[1]) {
pass.ReportRangef(call, "avoid using reflect.DeepEqual with errors")
return nil, nil
var errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
// hasError reports whether the type of e contains the type error.
// See containsError, below, for the meaning of "contains".
func hasError(pass *analysis.Pass, e ast.Expr) bool {
tv, ok := pass.TypesInfo.Types[e]
if !ok { // no type info, assume good
return false
return containsError(tv.Type)
// Report whether any type that typ could store and that could be compared is the
// error type. This includes typ itself, as well as the types of struct field, slice
// and array elements, map keys and elements, and pointers. It does not include
// channel types (incomparable), arg and result types of a Signature (not stored), or
// methods of a named or interface type (not stored).
func containsError(typ types.Type) bool {
// Track types being processed, to avoid infinite recursion.
// Using types as keys here is OK because we are checking for the identical pointer, not
// type identity. See analysis/passes/printf/types.go.
inProgress := make(map[types.Type]bool)
var check func(t types.Type) bool
check = func(t types.Type) bool {
if t == errorType {
return true
if inProgress[t] {
return false
inProgress[t] = true
switch t := t.(type) {
case *types.Pointer:
return check(t.Elem())
case *types.Slice:
return check(t.Elem())
case *types.Array:
return check(t.Elem())
case *types.Map:
return check(t.Key()) || check(t.Elem())
case *types.Struct:
for i := 0; i < t.NumFields(); i++ {
if check(t.Field(i).Type()) {
return true
case *types.Named:
return check(t.Underlying())
// We list the remaining valid type kinds for completeness.
case *types.Basic:
case *types.Chan: // channels store values, but they are not comparable
case *types.Signature:
case *types.Tuple: // tuples are only part of signatures
case *types.Interface:
return false
return check(typ)