reflect: unexported fields are tied to a package

An unexported field of a struct is not visible outside of the package
that defines it, so the package path is implicitly part of the
definition of any struct with an unexported field.

Change-Id: I17c6aac822bd0c24188ab8ba1cc406d6b5d82771
Reviewed-on: https://go-review.googlesource.com/32820
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -5913,3 +5913,15 @@ func TestSwapper(t *testing.T) {
 		}
 	}
 }
+
+func TestInaccessibleField(t *testing.T) {
+	var b Buffer
+	var localBuffer struct {
+		buf []byte
+	}
+	lv := ValueOf(&localBuffer).Elem()
+	rv := ValueOf(b)
+	shouldPanic(func() {
+		lv.Set(rv)
+	})
+}

src/reflect/export_test.go

@@ -113,3 +113,7 @@ func IsExported(t Type) bool {
 func ResolveReflectName(s string) {
 	resolveReflectName(newName(s, "", "", false))
 }
+
+type Buffer struct {
+	buf []byte
+}

src/reflect/type.go

@@ -1680,6 +1680,7 @@ func haveIdenticalUnderlyingType(T, V *rtype, cmpTags bool) bool {
 		if len(t.fields) != len(v.fields) {
 			return false
 		}
+		allExported := true
 		for i := range t.fields {
 			tf := &t.fields[i]
 			vf := &v.fields[i]
@@ -1695,6 +1696,15 @@ func haveIdenticalUnderlyingType(T, V *rtype, cmpTags bool) bool {
 			if tf.offset != vf.offset {
 				return false
 			}
+			allExported = allExported && tf.name.isExported()
+		}
+		if !allExported && t.pkgPath.name() != v.pkgPath.name() {
+			// An unexported field of a struct is not
+			// visible outside of the package that defines
+			// it, so the package path is implicitly part
+			// of the definition of any struct with an
+			// unexported field.
+			return false
 		}
 		return true
 	}