From d64847f1acae190202750856fa522991d86d07ed Mon Sep 17 00:00:00 2001
From: polyfloyd <floyd@polyfloyd.net>
Date: Fri, 9 Sep 2022 14:00:33 +0200
Subject: [PATCH] Permit comparisons in `(*t).Is(error) bool` functions (fixes
 #11)

---
 errorlint/analysis.go                      | 15 +++++++++-
 errorlint/lint.go                          | 33 +++++++++++++++++++-
 errorlint/testdata/src/issues/github-11.go | 35 ++++++++++++++++++++++
 3 files changed, 81 insertions(+), 2 deletions(-)
 create mode 100644 errorlint/testdata/src/issues/github-11.go

diff --git a/errorlint/analysis.go b/errorlint/analysis.go
index 58ddb26..c24d1f4 100644
--- a/errorlint/analysis.go
+++ b/errorlint/analysis.go
@@ -57,7 +57,7 @@ func run(pass *analysis.Pass) (interface{}, error) {
 type TypesInfoExt struct {
 	types.Info
 
-	// Maps AST nodes back to the node they are contain within.
+	// Maps AST nodes back to the node they are contained within.
 	NodeParent map[ast.Node]ast.Node
 
 	// Maps an object back to all identifiers to refer to it.
@@ -97,3 +97,16 @@ func newTypesInfoExt(info *types.Info) *TypesInfoExt {
 		IdentifiersForObject: identifiersForObject,
 	}
 }
+
+func (info *TypesInfoExt) ContainingFuncDecl(node ast.Node) *ast.FuncDecl {
+	for parent := info.NodeParent[node]; ; parent = info.NodeParent[parent] {
+		if _, ok := parent.(*ast.File); ok {
+			break
+		}
+		if fun, ok := parent.(*ast.FuncDecl); ok {
+			return fun
+			break
+		}
+	}
+	return nil
+}
diff --git a/errorlint/lint.go b/errorlint/lint.go
index 5301a3f..52d7a82 100644
--- a/errorlint/lint.go
+++ b/errorlint/lint.go
@@ -156,10 +156,14 @@ func LintErrorComparisons(fset *token.FileSet, info *TypesInfoExt) []Lint {
 		if !isErrorComparison(info.Info, binExpr) {
 			continue
 		}
-
+		// Some errors that are returned from some functions are exempt.
 		if isAllowedErrorComparison(info, binExpr) {
 			continue
 		}
+		// Comparisons that happen in `func (type) Is(error) bool` are okay.
+		if isNodeInErrorIsFunc(info, binExpr) {
+			continue
+		}
 
 		lints = append(lints, Lint{
 			Message: fmt.Sprintf("comparing with %s will fail on wrapped errors. Use errors.Is to check for a specific error", binExpr.Op),
@@ -181,6 +185,9 @@ func LintErrorComparisons(fset *token.FileSet, info *TypesInfoExt) []Lint {
 		if tagType.Type.String() != "error" {
 			continue
 		}
+		if isNodeInErrorIsFunc(info, switchStmt) {
+			continue
+		}
 
 		lints = append(lints, Lint{
 			Message: "switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors",
@@ -207,6 +214,30 @@ func isErrorComparison(info types.Info, binExpr *ast.BinaryExpr) bool {
 	return tx.Type.String() == "error" || ty.Type.String() == "error"
 }
 
+func isNodeInErrorIsFunc(info *TypesInfoExt, node ast.Node) bool {
+	funcDecl := info.ContainingFuncDecl(node)
+	if funcDecl == nil {
+		return false
+	}
+
+	if funcDecl.Name.Name != "Is" {
+		return false
+	}
+	if funcDecl.Recv == nil {
+		return false
+	}
+	// There should be 1 argument of type error.
+	if ii := funcDecl.Type.Params.List; len(ii) != 1 || info.Types[ii[0].Type].Type.String() != "error" {
+		return false
+	}
+	// The return type should be bool.
+	if ii := funcDecl.Type.Results.List; len(ii) != 1 || info.Types[ii[0].Type].Type.String() != "bool" {
+		return false
+	}
+
+	return true
+}
+
 func LintErrorTypeAssertions(fset *token.FileSet, info types.Info) []Lint {
 	lints := []Lint{}
 
diff --git a/errorlint/testdata/src/issues/github-11.go b/errorlint/testdata/src/issues/github-11.go
new file mode 100644
index 0000000..6d19ade
--- /dev/null
+++ b/errorlint/testdata/src/issues/github-11.go
@@ -0,0 +1,35 @@
+package issues
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+// Regression test for: https://github.com/polyfloyd/go-errorlint/issues/11
+
+var ErrInvalidConfig = errors.New("invalid configuration")
+
+// invalidConfig type is needed to make any error returned from Validator
+// to be ErrInvalidConfig.
+type invalidConfig struct {
+	err error
+}
+
+func (e *invalidConfig) Is(target error) bool {
+	return target == ErrInvalidConfig
+}
+
+type Errno uintptr
+
+func (e Errno) Is(target error) bool {
+	switch target {
+	case os.ErrPermission:
+		return e == Errno(syscall.EACCES) || e == Errno(syscall.EPERM)
+	case os.ErrExist:
+		return e == Errno(syscall.EEXIST) || e == Errno(syscall.ENOTEMPTY)
+	case os.ErrNotExist:
+		return e == Errno(syscall.ENOENT)
+	}
+	return false
+}