fix: Also apply allowed list to value switch statements

This also moves the reported AST position of switch statements from the
switch keyword to the first problematic case.

errorlint/allowed.go

@@ -127,13 +127,13 @@ func isAllowedErrAndFunc(err, fun string) bool {
 	return false
 }
 
-func isAllowedErrorComparison(pass *TypesInfoExt, binExpr *ast.BinaryExpr) bool {
+func isAllowedErrorComparison(pass *TypesInfoExt, a, b ast.Expr) bool {
 	var errName string // `<package>.<name>`, e.g. `io.EOF`
 	var callExprs []*ast.CallExpr
 
 	// Figure out which half of the expression is the returned error and which
 	// half is the presumed error declaration.
-	for _, expr := range []ast.Expr{binExpr.X, binExpr.Y} {
+	for _, expr := range []ast.Expr{a, b} {
 		switch t := expr.(type) {
 		case *ast.SelectorExpr:
 			// A selector which we assume refers to a staticaly declared error

errorlint/lint.go

@@ -172,15 +172,15 @@ func LintErrorComparisons(info *TypesInfoExt) []analysis.Diagnostic {
 			continue
 		}
 		// Comparing errors with nil is okay.
-		if isNilComparison(binExpr) {
+		if isNil(binExpr.X) || isNil(binExpr.Y) {
 			continue
 		}
 		// Find comparisons of which one side is a of type error.
-		if !isErrorComparison(info.TypesInfo, binExpr) {
+		if !isErrorType(info.TypesInfo, binExpr.X) && !isErrorType(info.TypesInfo, binExpr.Y) {
 			continue
 		}
 		// Some errors that are returned from some functions are exempt.
-		if isAllowedErrorComparison(info, binExpr) {
+		if isAllowedErrorComparison(info, binExpr.X, binExpr.Y) {
 			continue
 		}
 		// Comparisons that happen in `func (type) Is(error) bool` are okay.
@@ -201,43 +201,52 @@ func LintErrorComparisons(info *TypesInfoExt) []analysis.Diagnostic {
 			continue
 		}
 		// Check whether the switch operates on an error type.
-		if switchStmt.Tag == nil {
+		if !isErrorType(info.TypesInfo, switchStmt.Tag) {
 			continue
 		}
-		tagType := info.TypesInfo.Types[switchStmt.Tag]
-		if tagType.Type.String() != "error" {
+
+		var problematicCaseClause *ast.CaseClause
+	outer:
+		for _, stmt := range switchStmt.Body.List {
+			caseClause := stmt.(*ast.CaseClause)
+			for _, caseExpr := range caseClause.List {
+				if isNil(caseExpr) {
+					continue
+				}
+				// Some errors that are returned from some functions are exempt.
+				if !isAllowedErrorComparison(info, switchStmt.Tag, caseExpr) {
+					problematicCaseClause = caseClause
+					break outer
+				}
+			}
+		}
+		if problematicCaseClause == nil {
 			continue
 		}
+		// Comparisons that happen in `func (type) Is(error) bool` are okay.
 		if isNodeInErrorIsFunc(info, switchStmt) {
 			continue
 		}
 
 		if switchComparesNonNil(switchStmt) {
 			lints = append(lints, analysis.Diagnostic{
 				Message: "switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors",
-				Pos:     switchStmt.Pos(),
+				Pos:     problematicCaseClause.Pos(),
 			})
 		}
-
 	}
 
 	return lints
 }
 
-func isNilComparison(binExpr *ast.BinaryExpr) bool {
-	if ident, ok := binExpr.X.(*ast.Ident); ok && ident.Name == "nil" {
-		return true
-	}
-	if ident, ok := binExpr.Y.(*ast.Ident); ok && ident.Name == "nil" {
-		return true
-	}
-	return false
+func isNil(ex ast.Expr) bool {
+	ident, ok := ex.(*ast.Ident)
+	return ok && ident.Name == "nil"
 }
 
-func isErrorComparison(info *types.Info, binExpr *ast.BinaryExpr) bool {
-	tx := info.Types[binExpr.X]
-	ty := info.Types[binExpr.Y]
-	return tx.Type.String() == "error" || ty.Type.String() == "error"
+func isErrorType(info *types.Info, ex ast.Expr) bool {
+	t := info.Types[ex].Type
+	return t != nil && t.String() == "error"
 }
 
 func isNodeInErrorIsFunc(info *TypesInfoExt, node ast.Node) bool {

errorlint/testdata/src/errorsis/errorsis.go

@@ -76,10 +76,10 @@ func NotEqualOperatorYoda() {
 
 func CompareSwitch() {
 	err := doThing()
-	switch err { // want `switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors`
+	switch err {
 	case nil:
 		fmt.Println("nil")
-	case ErrFoo:
+	case ErrFoo: // want `switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors`
 		fmt.Println("ErrFoo")
 	}
 }
@@ -95,8 +95,8 @@ func CompareSwitchSafe() {
 }
 
 func CompareSwitchInline() {
-	switch doThing() { // want `switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors`
-	case ErrFoo:
+	switch doThing() {
+	case ErrFoo: // want `switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors`
 		fmt.Println("ErrFoo")
 	}
 }

errorlint/testdata/src/issues/github-54.go

@@ -0,0 +1,18 @@
+package issues
+
+import (
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+func SwitchOnUnixErrors() {
+	err := unix.Rmdir("somepath")
+	switch err {
+	case unix.ENOENT:
+		return
+	case unix.EPERM:
+		return
+	}
+	fmt.Println(err)
+}