fix crash for fields of built-in type

internal/tests/ftypes/input.go

@@ -0,0 +1,13 @@
+// Copyright 2017 Felix Lange <fjl@twurst.com>.
+// Use of this source code is governed by the MIT license,
+// which can be found in the LICENSE file.
+
+//go:generate gencodec -type X -formats json -out output.go
+
+package ftypes
+
+type X struct {
+	Int int
+	Err error
+	If  interface{}
+}

internal/tests/ftypes/output.go

@@ -0,0 +1,42 @@
+// Code generated by github.com/fjl/gencodec. DO NOT EDIT.
+
+package ftypes
+
+import (
+	"encoding/json"
+)
+
+func (x X) MarshalJSON() ([]byte, error) {
+	type X struct {
+		Int int
+		Err error
+		If  interface{}
+	}
+	var enc X
+	enc.Int = x.Int
+	enc.Err = x.Err
+	enc.If = x.If
+	return json.Marshal(&enc)
+}
+
+func (x *X) UnmarshalJSON(input []byte) error {
+	type X struct {
+		Int *int
+		Err *error
+		If  *interface{}
+	}
+	var dec X
+	if err := json.Unmarshal(input, &dec); err != nil {
+		return err
+	}
+	if dec.Int != nil {
+		x.Int = *dec.Int
+	}
+	if dec.Err != nil {
+		x.Err = *dec.Err
+	}
+	if dec.If != nil {
+		x.If = *dec.If
+	}
+	return nil
+}

main_test.go

@@ -24,6 +24,7 @@ func TestGolden(t *testing.T) {
 		Config{Dir: "nameclash", Type: "Y", FieldOverride: "Yo", Formats: AllFormats},
 		Config{Dir: "omitempty", Type: "X", FieldOverride: "Xo", Formats: AllFormats},
 		Config{Dir: "reqfield", Type: "X", Formats: []string{"json"}},
+		Config{Dir: "ftypes", Type: "X", Formats: []string{"json"}},
 		Config{Dir: "funcoverride", Type: "Z", FieldOverride: "Zo", Formats: AllFormats},
 	}
 	for _, test := range tests {

typeutil.go

@@ -32,6 +32,10 @@ func walkNamedTypes(typ types.Type, callback func(*types.Named)) {
 		for i := 0; i < typ.NumFields(); i++ {
 			walkNamedTypes(typ.Field(i).Type(), callback)
 		}
+	case *types.Interface:
+		if typ.NumMethods() > 0 {
+			panic("BUG: can't walk non-empty interface")
+		}
 	default:
 		panic(fmt.Errorf("BUG: can't walk %T", typ))
 	}
@@ -176,9 +180,10 @@ func (s *fileScope) addImport(path string) {
 // addReferences marks all names referenced by typ as used.
 func (s *fileScope) addReferences(typ types.Type) {
 	walkNamedTypes(typ, func(nt *types.Named) {
-		if nt.Obj().Pkg() == s.pkg {
+		pkg := nt.Obj().Pkg()
+		if pkg == s.pkg {
 			s.otherNames[nt.Obj().Name()] = true
-		} else {
+		} else if pkg != nil {
 			s.insertImport(nt.Obj().Pkg())
 		}
 	})