Add support for concrete error types. Fixes kisielk#80

internal/errcheck/errcheck.go

@@ -20,6 +20,13 @@ import (
 	"golang.org/x/tools/go/types"
 )
 
+var errorType *types.Interface
+
+func init() {
+	errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
+
+}
+
 var (
 	// ErrNoGoFiles is returned when CheckPackage is run on a package with no Go source files
 	ErrNoGoFiles = errors.New("package contains no go source files")
@@ -372,10 +379,9 @@ func (v *visitor) Visit(node ast.Node) ast.Visitor {
 }
 
 type obj interface {
-	Pkg() *types.Package
-	Name() string
+	Type() types.Type
 }
 
 func isErrorType(v obj) bool {
-	return v.Pkg() == nil && v.Name() == "error"
+	return types.Implements(v.Type(), errorType)
 }

internal/errcheck/errcheck_test.go

@@ -1,6 +1,7 @@
 package errcheck
 
 import (
+	"fmt"
 	"go/build"
 	"go/parser"
 	"go/token"
@@ -19,6 +20,14 @@ type marker struct {
 	line int
 }
 
+func newMarker(e UncheckedError) marker {
+	return marker{e.Pos.Filename, e.Pos.Line}
+}
+
+func (m marker) String() string {
+	return fmt.Sprintf("%s:%d", m.file, m.line)
+}
+
 func init() {
 	unchecked = make(map[marker]bool)
 	blank = make(map[marker]bool)
@@ -61,8 +70,14 @@ func TestUnchecked(t *testing.T) {
 	numErrors := len(unchecked)
 	if len(uerr.Errors) != numErrors {
 		t.Errorf("got %d errors, want %d", len(uerr.Errors), numErrors)
-		for i, err := range uerr.Errors {
-			t.Errorf("%d: %v", i, err)
+	unchecked_loop:
+		for k := range unchecked {
+			for _, e := range uerr.Errors {
+				if newMarker(e) == k {
+					continue unchecked_loop
+				}
+			}
+			t.Errorf("Expected unchecked at %s", k)
 		}
 		return
 	}
@@ -90,8 +105,23 @@ func TestBlank(t *testing.T) {
 	numErrors := len(unchecked) + len(blank)
 	if len(uerr.Errors) != numErrors {
 		t.Errorf("got %d errors, want %d", len(uerr.Errors), numErrors)
-		for i, err := range uerr.Errors {
-			t.Errorf("%d: %v", i, err)
+	unchecked_loop:
+		for k := range unchecked {
+			for _, e := range uerr.Errors {
+				if newMarker(e) == k {
+					continue unchecked_loop
+				}
+			}
+			t.Errorf("Expected unchecked at %s", k)
+		}
+	blank_loop:
+		for k := range blank {
+			for _, e := range uerr.Errors {
+				if newMarker(e) == k {
+					continue blank_loop
+				}
+			}
+			t.Errorf("Expected blank at %s", k)
 		}
 		return
 	}

main_test.go

@@ -54,7 +54,7 @@ func TestMain(t *testing.T) {
 		t.Errorf("Exit code is %d, expected %d", exitCode, exitUncheckedError)
 	}
 
-	expectUnchecked := 9
+	expectUnchecked := 10
 	if got := strings.Count(out, "UNCHECKED"); got != expectUnchecked {
 		t.Errorf("Got %d UNCHECKED errors, expected %d in:\n%s", got, expectUnchecked, out)
 	}

testdata/main.go

@@ -25,6 +25,20 @@ func rec() {
 	defer recover() // UNCHECKED
 }
 
+type MyError string
+
+func (e MyError) Error() string {
+	return string(e)
+}
+
+func customError() error {
+	return MyError("an error occurred")
+}
+
+func customConcreteError() MyError {
+	return MyError("an error occurred")
+}
+
 func main() {
 	// Single error return
 	_ = a() // BLANK
@@ -34,6 +48,14 @@ func main() {
 	_, _ = b() // BLANK
 	b()        // UNCHECKED
 
+	// Return a custom error type
+	_ = customError() // BLANK
+	customError()     // UNCHECKED
+
+	// Return a custom concrete error type
+	_ = customConcreteError() // BLANK
+	customConcreteError()     // UNCHECKED
+
 	// Method with a single error return
 	x := t{}
 	_ = x.a() // BLANK