explicitly catch attempt to decode into a value - must be a pointer to see the result.

R=rsc
https://golang.org/cl/163070

src/pkg/gob/decoder.go

@@ -58,6 +58,13 @@ func (dec *Decoder) recvType(id typeId) {
 // The value underlying e must be the correct type for the next
 // data item received.
 func (dec *Decoder) Decode(e interface{}) os.Error {
+	// If e represents a value, the answer won't get back to the
+	// caller.  Make sure it's a pointer.
+	if _, ok := reflect.Typeof(e).(*reflect.PtrType); !ok {
+		dec.state.err = os.ErrorString("gob: attempt to decode into a non-pointer");
+		return dec.state.err;
+	}
+
 	// Make sure we're single-threaded through here.
 	dec.mutex.Lock();
 	defer dec.mutex.Unlock();

src/pkg/gob/encoder_test.go

@@ -9,6 +9,7 @@ import (
 	"io";
 	"os";
 	"reflect";
+	"strings";
 	"testing";
 )
 
@@ -195,7 +196,6 @@ func TestPtrTypeToType(t *testing.T) {
 }
 
 func TestTypeToPtrPtrPtrPtrType(t *testing.T) {
-	// Encode a *T, decode a T
 	type Type2 struct {
 		a ****float;
 	}
@@ -215,7 +215,6 @@ func TestTypeToPtrPtrPtrPtrType(t *testing.T) {
 }
 
 func TestSlice(t *testing.T) {
-	// Encode a *T, decode a T
 	type Type3 struct {
 		a []string;
 	}
@@ -225,3 +224,15 @@ func TestSlice(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestValueError(t *testing.T) {
+	// Encode a *T, decode a T
+	type Type4 struct {
+		a int;
+	}
+	t4p := Type4{3};	// note: not a pointer, unlike the other tests.
+	var t4 Type4;
+	if err := encAndDec(t4, t4p); err == nil || strings.Index(err.String(), "pointer") <= 0 {
+		t.Error("expected error; got none or got wrong one")
+	}
+}