[dev.typealias] reflect: fix StructOf use of StructField to match StructField docs

The runtime internal structField interprets name=="" as meaning anonymous,
but the exported reflect.StructField has always set Name, even for anonymous
fields, and also set Anonymous=true.

The initial implementation of StructOf confused the internal and public
meanings of the StructField, expecting the runtime representation of
anonymous fields instead of the exported reflect API representation.
It also did not document this fact, so that users had no way to know how
to create an anonymous field.

This CL changes StructOf to use the previously documented interpretation
of reflect.StructField instead of an undocumented one.

The implementation of StructOf also, in some cases, allowed creating
structs with unexported fields (if you knew how to ask) but set the
PkgPath incorrectly on those fields. Rather than try to fix that, this CL
changes StructOf to reject attempts to create unexported fields.
(I think that may be the right design choice, not just a temporary limitation.
In any event, it's not the topic for today's work.)

For #17766.
Fixes #18780.

Change-Id: I585a4e324dc5a90551f49d21ae04d2de9ea04b6c
Reviewed-on: https://go-review.googlesource.com/35731
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

2 files changed, 87 insertions, 74 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 033a18171d..e057b0cfcc 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -3689,7 +3689,7 @@ func checkSameType(t *testing.T, x, y interface{}) {
 
 func TestArrayOf(t *testing.T) {
 	// check construction and use of type not in binary
-	for _, table := range []struct {
+	tests := []struct {
 		n          int
 		value      func(i int) interface{}
 		comparable bool
@@ -3767,7 +3767,9 @@ func TestArrayOf(t *testing.T) {
 			comparable: true,
 			want:       "[{0 0} {1 1} {2 2} {3 3} {4 4} {5 5} {6 6} {7 7} {8 8} {9 9}]",
 		},
-	} {
+	}
+
+	for _, table := range tests {
 		at := ArrayOf(table.n, TypeOf(table.value(0)))
 		v := New(at).Elem()
 		vok := New(at).Elem()
@@ -4133,50 +4135,58 @@ func TestStructOfExportRules(t *testing.T) {
 		f()
 	}
 
-	for i, test := range []struct {
+	tests := []struct {
 		field     StructField
 		mustPanic bool
 		exported  bool
 	}{
 		{
-			field:     StructField{Name: "", Type: TypeOf(S1{})},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S1", Anonymous: true, Type: TypeOf(S1{})},
+			exported: true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf((*S1)(nil))},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S1", Anonymous: true, Type: TypeOf((*S1)(nil))},
+			exported: true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf(s2{})},
-			mustPanic: false,
-			exported:  false,
+			field:     StructField{Name: "s2", Anonymous: true, Type: TypeOf(s2{})},
+			mustPanic: true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf((*s2)(nil))},
-			mustPanic: false,
-			exported:  false,
+			field:     StructField{Name: "s2", Anonymous: true, Type: TypeOf((*s2)(nil))},
+			mustPanic: true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf(S1{}), PkgPath: "other/pkg"},
+			field:     StructField{Name: "Name", Type: nil, PkgPath: ""},
 			mustPanic: true,
-			exported:  true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf((*S1)(nil)), PkgPath: "other/pkg"},
+			field:     StructField{Name: "", Type: TypeOf(S1{}), PkgPath: ""},
+			mustPanic: true,
+		},
+		{
+			field:     StructField{Name: "S1", Anonymous: true, Type: TypeOf(S1{}), PkgPath: "other/pkg"},
+			mustPanic: true,
+		},
+		{
+			field:     StructField{Name: "S1", Anonymous: true, Type: TypeOf((*S1)(nil)), PkgPath: "other/pkg"},
+			mustPanic: true,
+		},
+		{
+			field:     StructField{Name: "s2", Anonymous: true, Type: TypeOf(s2{}), PkgPath: "other/pkg"},
 			mustPanic: true,
-			exported:  true,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf(s2{}), PkgPath: "other/pkg"},
+			field:     StructField{Name: "s2", Anonymous: true, Type: TypeOf((*s2)(nil)), PkgPath: "other/pkg"},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
-			field:     StructField{Name: "", Type: TypeOf((*s2)(nil)), PkgPath: "other/pkg"},
+			field:     StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
+			mustPanic: true,
+		},
+		{
+			field:     StructField{Name: "s2", Type: TypeOf(int(0)), PkgPath: "other/pkg"},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "S", Type: TypeOf(S1{})},
@@ -4184,81 +4194,68 @@ func TestStructOfExportRules(t *testing.T) {
 			exported:  true,
 		},
 		{
-			field:     StructField{Name: "S", Type: TypeOf((*S1)(nil))},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S", Type: TypeOf((*S1)(nil))},
+			exported: true,
 		},
 		{
-			field:     StructField{Name: "S", Type: TypeOf(s2{})},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S", Type: TypeOf(s2{})},
+			exported: true,
 		},
 		{
-			field:     StructField{Name: "S", Type: TypeOf((*s2)(nil))},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "S", Type: TypeOf((*s2)(nil))},
+			exported: true,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf(S1{})},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf((*S1)(nil))},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf(s2{})},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf((*s2)(nil))},
 			mustPanic: true,
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf(S1{}), PkgPath: "other/pkg"},
 			mustPanic: true, // TODO(sbinet): creating a name with a package path
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf((*S1)(nil)), PkgPath: "other/pkg"},
 			mustPanic: true, // TODO(sbinet): creating a name with a package path
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf(s2{}), PkgPath: "other/pkg"},
 			mustPanic: true, // TODO(sbinet): creating a name with a package path
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "s", Type: TypeOf((*s2)(nil)), PkgPath: "other/pkg"},
 			mustPanic: true, // TODO(sbinet): creating a name with a package path
-			exported:  false,
 		},
 		{
 			field:     StructField{Name: "", Type: TypeOf(ΦType{})},
-			mustPanic: false,
-			exported:  true,
+			mustPanic: true,
 		},
 		{
 			field:     StructField{Name: "", Type: TypeOf(φType{})},
-			mustPanic: false,
-			exported:  false,
+			mustPanic: true,
 		},
 		{
-			field:     StructField{Name: "Φ", Type: TypeOf(0)},
-			mustPanic: false,
-			exported:  true,
+			field:    StructField{Name: "Φ", Type: TypeOf(0)},
+			exported: true,
 		},
 		{
-			field:     StructField{Name: "φ", Type: TypeOf(0)},
-			mustPanic: false,
-			exported:  false,
+			field:    StructField{Name: "φ", Type: TypeOf(0)},
+			exported: false,
 		},
-	} {
+	}
+
+	for i, test := range tests {
 		testPanic(i, test.mustPanic, func() {
 			typ := StructOf([]StructField{test.field})
 			if typ == nil {
@@ -4346,7 +4343,7 @@ func TestStructOfGenericAlg(t *testing.T) {
 		{Name: "S1", Type: st1},
 	})
 
-	for _, table := range []struct {
+	tests := []struct {
 		rt  Type
 		idx []int
 	}{
@@ -4427,7 +4424,9 @@ func TestStructOfGenericAlg(t *testing.T) {
 			),
 			idx: []int{2},
 		},
-	} {
+	}
+
+	for _, table := range tests {
 		v1 := New(table.rt).Elem()
 		v2 := New(table.rt).Elem()
 
@@ -4529,18 +4528,21 @@ func TestStructOfWithInterface(t *testing.T) {
 	type Iface interface {
 		Get() int
 	}
-	for i, table := range []struct {
+	tests := []struct {
+		name string
 		typ  Type
 		val  Value
 		impl bool
 	}{
 		{
+			name: "StructI",
 			typ:  TypeOf(StructI(want)),
 			val:  ValueOf(StructI(want)),
 			impl: true,
 		},
 		{
-			typ: PtrTo(TypeOf(StructI(want))),
+			name: "StructI",
+			typ:  PtrTo(TypeOf(StructI(want))),
 			val: ValueOf(func() interface{} {
 				v := StructI(want)
 				return &v
@@ -4548,7 +4550,8 @@ func TestStructOfWithInterface(t *testing.T) {
 			impl: true,
 		},
 		{
-			typ: PtrTo(TypeOf(StructIPtr(want))),
+			name: "StructIPtr",
+			typ:  PtrTo(TypeOf(StructIPtr(want))),
 			val: ValueOf(func() interface{} {
 				v := StructIPtr(want)
 				return &v
@@ -4556,6 +4559,7 @@ func TestStructOfWithInterface(t *testing.T) {
 			impl: true,
 		},
 		{
+			name: "StructIPtr",
 			typ:  TypeOf(StructIPtr(want)),
 			val:  ValueOf(StructIPtr(want)),
 			impl: false,
@@ -4565,13 +4569,16 @@ func TestStructOfWithInterface(t *testing.T) {
 		//	val:  ValueOf(StructI(want)),
 		//	impl: true,
 		// },
-	} {
+	}
+
+	for i, table := range tests {
 		rt := StructOf(
 			[]StructField{
 				{
-					Name:    "",
-					PkgPath: "",
-					Type:    table.typ,
+					Name:      table.name,
+					Anonymous: true,
+					PkgPath:   "",
+					Type:      table.typ,
 				},
 			},
 		)
@@ -5951,6 +5958,7 @@ func TestSwapper(t *testing.T) {
 			want: []pairPtr{{5, 6, &c}, {3, 4, &b}, {1, 2, &a}},
 		},
 	}
+
 	for i, tt := range tests {
 		inStr := fmt.Sprint(tt.in)
 		Swapper(tt.in)(tt.i, tt.j)
diff --git a/src/reflect/type.go b/src/reflect/type.go
index 9d6e7a6846..e0be36d970 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -2403,6 +2403,9 @@ func StructOf(fields []StructField) Type {
 	lastzero := uintptr(0)
 	repr = append(repr, "struct {"...)
 	for i, field := range fields {
+		if field.Name == "" {
+			panic("reflect.StructOf: field " + strconv.Itoa(i) + " has no name")
+		}
 		if field.Type == nil {
 			panic("reflect.StructOf: field " + strconv.Itoa(i) + " has no type")
 		}
@@ -2794,23 +2797,25 @@ func StructOf(fields []StructField) Type {
 }
 
 func runtimeStructField(field StructField) structField {
-	exported := field.PkgPath == ""
-	if field.Name == "" {
-		t := field.Type.(*rtype)
-		if t.Kind() == Ptr {
-			t = t.Elem().(*rtype)
-		}
-		exported = t.nameOff(t.str).isExported()
-	} else if exported {
-		b0 := field.Name[0]
-		if ('a' <= b0 && b0 <= 'z') || b0 == '_' {
-			panic("reflect.StructOf: field \"" + field.Name + "\" is unexported but has no PkgPath")
-		}
+	if field.PkgPath != "" {
+		panic("reflect.StructOf: StructOf does not allow unexported fields")
+	}
+
+	// Best-effort check for misuse.
+	// Since PkgPath is empty, not much harm done if Unicode lowercase slips through.
+	c := field.Name[0]
+	if 'a' <= c && c <= 'z' || c == '_' {
+		panic("reflect.StructOf: field \"" + field.Name + "\" is unexported but missing PkgPath")
+	}
+
+	name := field.Name
+	if field.Anonymous {
+		name = ""
 	}
 
-	_ = resolveReflectType(field.Type.common())
+	resolveReflectType(field.Type.common()) // install in runtime
 	return structField{
-		name:   newName(field.Name, string(field.Tag), field.PkgPath, exported),
+		name:   newName(name, string(field.Tag), "", true),
 		typ:    field.Type.common(),
 		offset: 0,
 	}