encoding/gob: better error messages when types mismatch

The transmitter must encode an interface value if it is to be decoded
into an interface value, but it's a common and confusing error to
encode a concrete value and attempt to decode it into an interface,
particularly *interface{}. This CL attempts to explain things better.

Fixes #2367.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5485072

src/pkg/encoding/gob/decode.go

@@ -1068,7 +1068,12 @@ func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *de
 	engine.instr = make([]decInstr, 1) // one item
 	name := rt.String()                // best we can do
 	if !dec.compatibleType(rt, remoteId, make(map[reflect.Type]typeId)) {
-		return nil, errors.New("gob: wrong type received for local value " + name + ": " + dec.typeString(remoteId))
+		remoteType := dec.typeString(remoteId)
+		// Common confusing case: local interface type, remote concrete type.
+		if ut.base.Kind() == reflect.Interface && remoteId != tInterface {
+			return nil, errors.New("gob: local interface type " + name + " can only be decoded from remote interface type; received concrete type " + remoteType)
+		}
+		return nil, errors.New("gob: decoding into local type " + name + ", received remote type " + remoteType)
 	}
 	op, indir := dec.decOpFor(remoteId, rt, name, make(map[reflect.Type]*decOp))
 	ovfl := errors.New(`value for "` + name + `" out of range`)

src/pkg/encoding/gob/encoder_test.go

@@ -309,7 +309,7 @@ var singleTests = []SingleTest{
 	{[7]int{4, 55, 1, 44, 22, 66, 1234}, &testArray, ""},
 
 	// Decode errors
-	{172, &testFloat32, "wrong type"},
+	{172, &testFloat32, "type"},
 }
 
 func TestSingletons(t *testing.T) {