reflect: make StructOf panic for methods that don't work

When StructOf is used with an anonymous field that has methods, and
that anonymous field is not the first field, the methods we generate
are incorrect because they do not offset to the field as required.
If we encounter that case, panic rather than doing the wrong thing.

Fixes #20824
Updates #15924

Change-Id: I3b0901ddbc6d58af5f7e84660b5e3085a431035d
Reviewed-on: https://go-review.googlesource.com/47035
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/all_test.go

@@ -4686,24 +4686,49 @@ func TestStructOfWithInterface(t *testing.T) {
 	}
 
 	for i, table := range tests {
-		rt := StructOf(
+		for j := 0; j < 2; j++ {
-			[]StructField{
+			var fields []StructField
-				{
+			if j == 1 {
+				fields = append(fields, StructField{
+					Name:    "Dummy",
+					PkgPath: "",
+					Type:    TypeOf(int(0)),
+				})
+			}
+			fields = append(fields, StructField{
 				Name:      table.name,
 				Anonymous: true,
 				PkgPath:   "",
 				Type:      table.typ,
-				},
+			})
-			},
+
-		)
+			// We currently do not correctly implement methods
+			// for anonymous fields other than the first.
+			// Therefore, for now, we expect those methods
+			// to not exist.  See issues 15924 and 20824.
+			// When those issues are fixed, this test of panic
+			// should be removed.
+			if j == 1 && table.impl {
+				func() {
+					defer func() {
+						if err := recover(); err == nil {
+							t.Errorf("test-%d-%d did not panic", i, j)
+						}
+					}()
+					_ = StructOf(fields)
+				}()
+				continue
+			}
+
+			rt := StructOf(fields)
 			rv := New(rt).Elem()
-		rv.Field(0).Set(table.val)
+			rv.Field(j).Set(table.val)
 
 			if _, ok := rv.Interface().(Iface); ok != table.impl {
 				if table.impl {
-				t.Errorf("test-%d: type=%v fails to implement Iface.\n", i, table.typ)
+					t.Errorf("test-%d-%d: type=%v fails to implement Iface.\n", i, j, table.typ)
 				} else {
-				t.Errorf("test-%d: type=%v should NOT implement Iface\n", i, table.typ)
+					t.Errorf("test-%d-%d: type=%v should NOT implement Iface\n", i, j, table.typ)
 				}
 				continue
 			}
@@ -4714,13 +4739,14 @@ func TestStructOfWithInterface(t *testing.T) {
 
 			v := rv.Interface().(Iface).Get()
 			if v != want {
-			t.Errorf("test-%d: x.Get()=%v. want=%v\n", i, v, want)
+				t.Errorf("test-%d-%d: x.Get()=%v. want=%v\n", i, j, v, want)
 			}
 
 			fct := rv.MethodByName("Get")
 			out := fct.Call(nil)
 			if !DeepEqual(out[0].Interface(), want) {
-			t.Errorf("test-%d: x.Get()=%v. want=%v\n", i, out[0].Interface(), want)
+				t.Errorf("test-%d-%d: x.Get()=%v. want=%v\n", i, j, out[0].Interface(), want)
+			}
 		}
 	}
 }

src/reflect/type.go

@@ -2434,7 +2434,7 @@ func StructOf(fields []StructField) Type {
 				ift := (*interfaceType)(unsafe.Pointer(ft))
 				for im, m := range ift.methods {
 					if ift.nameOff(m.name).pkgPath() != "" {
-						// TODO(sbinet)
+						// TODO(sbinet).  Issue 15924.
 						panic("reflect: embedded interface with unexported method(s) not implemented")
 					}
 
@@ -2492,10 +2492,15 @@ func StructOf(fields []StructField) Type {
 			case Ptr:
 				ptr := (*ptrType)(unsafe.Pointer(ft))
 				if unt := ptr.uncommon(); unt != nil {
+					if i > 0 && unt.mcount > 0 {
+						// Issue 15924.
+						panic("reflect: embedded type with methods not implemented if type is not first field")
+					}
 					for _, m := range unt.methods() {
 						mname := ptr.nameOff(m.name)
 						if mname.pkgPath() != "" {
-							// TODO(sbinet)
+							// TODO(sbinet).
+							// Issue 15924.
 							panic("reflect: embedded interface with unexported method(s) not implemented")
 						}
 						methods = append(methods, method{
@@ -2511,6 +2516,7 @@ func StructOf(fields []StructField) Type {
 						mname := ptr.nameOff(m.name)
 						if mname.pkgPath() != "" {
 							// TODO(sbinet)
+							// Issue 15924.
 							panic("reflect: embedded interface with unexported method(s) not implemented")
 						}
 						methods = append(methods, method{
@@ -2523,10 +2529,15 @@ func StructOf(fields []StructField) Type {
 				}
 			default:
 				if unt := ft.uncommon(); unt != nil {
+					if i > 0 && unt.mcount > 0 {
+						// Issue 15924.
+						panic("reflect: embedded type with methods not implemented if type is not first field")
+					}
 					for _, m := range unt.methods() {
 						mname := ft.nameOff(m.name)
 						if mname.pkgPath() != "" {
 							// TODO(sbinet)
+							// Issue 15924.
 							panic("reflect: embedded interface with unexported method(s) not implemented")
 						}
 						methods = append(methods, method{