reflect: add Method.IsExported and StructField.IsExported methods

The IsExported method is a more intuitive helper for checking whether
the method or field is exported than checking whether PkgPath is empty.

In the same CL, modify the standard library to make use of this helper.

Fixes #41563

Change-Id: Iaacfb3b74449501f98e2707aa32095a32bd3c3c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/266197
Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

8 files changed, 68 insertions, 21 deletions

diff --git a/src/encoding/asn1/asn1.go b/src/encoding/asn1/asn1.go
index f9b9cb4930..cffc06dc9c 100644
--- a/src/encoding/asn1/asn1.go
+++ b/src/encoding/asn1/asn1.go
@@ -914,7 +914,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 		structType := fieldType
 
 		for i := 0; i < structType.NumField(); i++ {
-			if structType.Field(i).PkgPath != "" {
+			if !structType.Field(i).IsExported() {
 				err = StructuralError{"struct contains unexported fields"}
 				return
 			}
diff --git a/src/encoding/asn1/marshal.go b/src/encoding/asn1/marshal.go
index 0d34d5aa1e..5b4d786d49 100644
--- a/src/encoding/asn1/marshal.go
+++ b/src/encoding/asn1/marshal.go
@@ -488,7 +488,7 @@ func makeBody(value reflect.Value, params fieldParameters) (e encoder, err error
 		t := v.Type()
 
 		for i := 0; i < t.NumField(); i++ {
-			if t.Field(i).PkgPath != "" {
+			if !t.Field(i).IsExported() {
 				return nil, StructuralError{"struct contains unexported fields"}
 			}
 		}
diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 483b9d8f2d..751f03d33d 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -1239,19 +1239,18 @@ func typeFields(t reflect.Type) structFields {
 			// Scan f.typ for fields to include.
 			for i := 0; i < f.typ.NumField(); i++ {
 				sf := f.typ.Field(i)
-				isUnexported := sf.PkgPath != ""
 				if sf.Anonymous {
 					t := sf.Type
 					if t.Kind() == reflect.Ptr {
 						t = t.Elem()
 					}
-					if isUnexported && t.Kind() != reflect.Struct {
+					if !sf.IsExported() && t.Kind() != reflect.Struct {
 						// Ignore embedded fields of unexported non-struct types.
 						continue
 					}
 					// Do not ignore embedded fields of unexported struct types
 					// since they may have exported fields.
-				} else if isUnexported {
+				} else if !sf.IsExported() {
 					// Ignore unexported non-embedded fields.
 					continue
 				}
diff --git a/src/encoding/xml/typeinfo.go b/src/encoding/xml/typeinfo.go
index f30fe58590..162724ef1a 100644
--- a/src/encoding/xml/typeinfo.go
+++ b/src/encoding/xml/typeinfo.go
@@ -60,7 +60,7 @@ func getTypeInfo(typ reflect.Type) (*typeInfo, error) {
 		n := typ.NumField()
 		for i := 0; i < n; i++ {
 			f := typ.Field(i)
-			if (f.PkgPath != "" && !f.Anonymous) || f.Tag.Get("xml") == "-" {
+			if (!f.IsExported() && !f.Anonymous) || f.Tag.Get("xml") == "-" {
 				continue // Private field
 			}
 
diff --git a/src/net/rpc/server.go b/src/net/rpc/server.go
index 9cb928240f..074c5b9b0d 100644
--- a/src/net/rpc/server.go
+++ b/src/net/rpc/server.go
@@ -283,7 +283,7 @@ func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
 		mtype := method.Type
 		mname := method.Name
 		// Method must be exported.
-		if method.PkgPath != "" {
+		if !method.IsExported() {
 			continue
 		}
 		// Method needs three ins: receiver, *args, *reply.
diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 1225d6177d..35cc469d74 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -2900,6 +2900,7 @@ func TestFieldPkgPath(t *testing.T) {
 		index    []int
 		pkgPath  string
 		embedded bool
+		exported bool
 	}
 
 	checkPkgPath := func(name string, s []pkgpathTest) {
@@ -2911,27 +2912,63 @@ func TestFieldPkgPath(t *testing.T) {
 			if got, want := f.Anonymous, test.embedded; got != want {
 				t.Errorf("%s: Field(%d).Anonymous = %v, want %v", name, test.index, got, want)
 			}
+			if got, want := f.IsExported(), test.exported; got != want {
+				t.Errorf("%s: Field(%d).IsExported = %v, want %v", name, test.index, got, want)
+			}
 		}
 	}
 
 	checkPkgPath("testStruct", []pkgpathTest{
-		{[]int{0}, "", false},             // Exported
-		{[]int{1}, "reflect_test", false}, // unexported
-		{[]int{2}, "", true},              // OtherPkgFields
-		{[]int{2, 0}, "", false},          // OtherExported
-		{[]int{2, 1}, "reflect", false},   // otherUnexported
-		{[]int{3}, "reflect_test", true},  // int
-		{[]int{4}, "reflect_test", true},  // *x
+		{[]int{0}, "", false, true},              // Exported
+		{[]int{1}, "reflect_test", false, false}, // unexported
+		{[]int{2}, "", true, true},               // OtherPkgFields
+		{[]int{2, 0}, "", false, true},           // OtherExported
+		{[]int{2, 1}, "reflect", false, false},   // otherUnexported
+		{[]int{3}, "reflect_test", true, false},  // int
+		{[]int{4}, "reflect_test", true, false},  // *x
 	})
 
 	type localOtherPkgFields OtherPkgFields
 	typ = TypeOf(localOtherPkgFields{})
 	checkPkgPath("localOtherPkgFields", []pkgpathTest{
-		{[]int{0}, "", false},        // OtherExported
-		{[]int{1}, "reflect", false}, // otherUnexported
+		{[]int{0}, "", false, true},         // OtherExported
+		{[]int{1}, "reflect", false, false}, // otherUnexported
 	})
 }
 
+func TestMethodPkgPath(t *testing.T) {
+	type I interface {
+		x()
+		X()
+	}
+	typ := TypeOf((*interface {
+		I
+		y()
+		Y()
+	})(nil)).Elem()
+
+	tests := []struct {
+		name     string
+		pkgPath  string
+		exported bool
+	}{
+		{"X", "", true},
+		{"Y", "", true},
+		{"x", "reflect_test", false},
+		{"y", "reflect_test", false},
+	}
+
+	for _, test := range tests {
+		m, _ := typ.MethodByName(test.name)
+		if got, want := m.PkgPath, test.pkgPath; got != want {
+			t.Errorf("MethodByName(%q).PkgPath = %q, want %q", test.name, got, want)
+		}
+		if got, want := m.IsExported(), test.exported; got != want {
+			t.Errorf("MethodByName(%q).IsExported = %v, want %v", test.name, got, want)
+		}
+	}
+}
+
 func TestVariadicType(t *testing.T) {
 	// Test example from Type documentation.
 	var f func(x int, y ...float64)
diff --git a/src/reflect/type.go b/src/reflect/type.go
index eb2030063a..dc235ea8f7 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -568,12 +568,13 @@ func newName(n, tag string, exported bool) name {
 // Method represents a single method.
 type Method struct {
 	// Name is the method name.
+	Name string
+
 	// PkgPath is the package path that qualifies a lower case (unexported)
 	// method name. It is empty for upper case (exported) method names.
 	// The combination of PkgPath and Name uniquely identifies a method
 	// in a method set.
 	// See https://golang.org/ref/spec#Uniqueness_of_identifiers
-	Name    string
 	PkgPath string
 
 	Type  Type  // method type
@@ -581,6 +582,11 @@ type Method struct {
 	Index int   // index for Type.Method
 }
 
+// IsExported reports whether the method is exported.
+func (m Method) IsExported() bool {
+	return m.PkgPath == ""
+}
+
 const (
 	kindDirectIface = 1 << 5
 	kindGCProg      = 1 << 6 // Type.gc points to GC program
@@ -1090,6 +1096,7 @@ func (t *interfaceType) MethodByName(name string) (m Method, ok bool) {
 type StructField struct {
 	// Name is the field name.
 	Name string
+
 	// PkgPath is the package path that qualifies a lower case (unexported)
 	// field name. It is empty for upper case (exported) field names.
 	// See https://golang.org/ref/spec#Uniqueness_of_identifiers
@@ -1102,6 +1109,11 @@ type StructField struct {
 	Anonymous bool      // is an embedded field
 }
 
+// IsExported reports whether the field is exported.
+func (f StructField) IsExported() bool {
+	return f.PkgPath == ""
+}
+
 // A StructTag is the tag string in a struct field.
 //
 // By convention, tag strings are a concatenation of
@@ -2771,8 +2783,7 @@ func runtimeStructField(field StructField) (structField, string) {
 		panic("reflect.StructOf: field \"" + field.Name + "\" is anonymous but has PkgPath set")
 	}
 
-	exported := field.PkgPath == ""
-	if exported {
+	if field.IsExported() {
 		// Best-effort check for misuse.
 		// Since this field will be treated as exported, not much harm done if Unicode lowercase slips through.
 		c := field.Name[0]
@@ -2788,7 +2799,7 @@ func runtimeStructField(field StructField) (structField, string) {
 
 	resolveReflectType(field.Type.common()) // install in runtime
 	f := structField{
-		name:        newName(field.Name, string(field.Tag), exported),
+		name:        newName(field.Name, string(field.Tag), field.IsExported()),
 		typ:         field.Type.common(),
 		offsetEmbed: offsetEmbed,
 	}
diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 19154fc640..4637b2035f 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -615,7 +615,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 		tField, ok := receiver.Type().FieldByName(fieldName)
 		if ok {
 			field := receiver.FieldByIndex(tField.Index)
-			if tField.PkgPath != "" { // field is unexported
+			if !tField.IsExported() {
 				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
 			}
 			// If it's a function, we must call it.