diff --git a/src/pkg/gob/codec_test.go b/src/pkg/gob/codec_test.go
index 848a871947..66d6b01ec5 100644
--- a/src/pkg/gob/codec_test.go
+++ b/src/pkg/gob/codec_test.go
@@ -7,6 +7,7 @@ package gob
 import (
 	"bytes";
 	"gob";
+	"math";
 	"os";
 	"reflect";
 	"strings";
@@ -347,21 +348,22 @@ func newdecodeState(data []byte) *decodeState {
 // Test instruction execution for decoding.
 // Do not run the machine yet; instead do individual instructions crafted by hand.
 func TestScalarDecInstructions(t *testing.T) {
+	ovfl := os.ErrorString("overflow");
 
 	// bool
 	{
 		var data struct { a bool };
-		instr := &decInstr{ decBool, 6, 0, 0 };
+		instr := &decInstr{ decBool, 6, 0, 0, ovfl };
 		state := newdecodeState(boolResult);
 		execDec("bool", instr, state, t, unsafe.Pointer(&data));
 		if data.a != true {
-			t.Errorf("int a = %v not true", data.a)
+			t.Errorf("bool a = %v not true", data.a)
 		}
 	}
 	// int
 	{
 		var data struct { a int };
-		instr := &decInstr{ decInt, 6, 0, 0 };
+		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
 		state := newdecodeState(signedResult);
 		execDec("int", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
@@ -372,139 +374,150 @@ func TestScalarDecInstructions(t *testing.T) {
 	// uint
 	{
 		var data struct { a uint };
-		instr := &decInstr{ decUint, 6, 0, 0 };
+		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
 		state := newdecodeState(unsignedResult);
 		execDec("uint", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("uint a = %v not 17", data.a)
 		}
 	}
 
 	// int8
 	{
 		var data struct { a int8 };
-		instr := &decInstr{ decInt8, 6, 0, 0 };
+		instr := &decInstr{ decInt8, 6, 0, 0, ovfl };
 		state := newdecodeState(signedResult);
 		execDec("int8", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("int8 a = %v not 17", data.a)
 		}
 	}
 
 	// uint8
 	{
 		var data struct { a uint8 };
-		instr := &decInstr{ decUint8, 6, 0, 0 };
+		instr := &decInstr{ decUint8, 6, 0, 0, ovfl };
 		state := newdecodeState(unsignedResult);
 		execDec("uint8", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("uint8 a = %v not 17", data.a)
 		}
 	}
 
 	// int16
 	{
 		var data struct { a int16 };
-		instr := &decInstr{ decInt16, 6, 0, 0 };
+		instr := &decInstr{ decInt16, 6, 0, 0, ovfl };
 		state := newdecodeState(signedResult);
 		execDec("int16", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("int16 a = %v not 17", data.a)
 		}
 	}
 
 	// uint16
 	{
 		var data struct { a uint16 };
-		instr := &decInstr{ decUint16, 6, 0, 0 };
+		instr := &decInstr{ decUint16, 6, 0, 0, ovfl };
 		state := newdecodeState(unsignedResult);
 		execDec("uint16", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("uint16 a = %v not 17", data.a)
 		}
 	}
 
 	// int32
 	{
 		var data struct { a int32 };
-		instr := &decInstr{ decInt32, 6, 0, 0 };
+		instr := &decInstr{ decInt32, 6, 0, 0, ovfl };
 		state := newdecodeState(signedResult);
 		execDec("int32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("int32 a = %v not 17", data.a)
 		}
 	}
 
 	// uint32
 	{
 		var data struct { a uint32 };
-		instr := &decInstr{ decUint32, 6, 0, 0 };
+		instr := &decInstr{ decUint32, 6, 0, 0, ovfl };
 		state := newdecodeState(unsignedResult);
 		execDec("uint32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("uint32 a = %v not 17", data.a)
+		}
+	}
+
+	// uintptr
+	{
+		var data struct { a uintptr };
+		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
+		state := newdecodeState(unsignedResult);
+		execDec("uintptr", instr, state, t, unsafe.Pointer(&data));
+		if data.a != 17 {
+			t.Errorf("uintptr a = %v not 17", data.a)
 		}
 	}
 
 	// int64
 	{
 		var data struct { a int64 };
-		instr := &decInstr{ decInt64, 6, 0, 0 };
+		instr := &decInstr{ decInt64, 6, 0, 0, ovfl };
 		state := newdecodeState(signedResult);
 		execDec("int64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("int64 a = %v not 17", data.a)
 		}
 	}
 
 	// uint64
 	{
 		var data struct { a uint64 };
-		instr := &decInstr{ decUint64, 6, 0, 0 };
+		instr := &decInstr{ decUint64, 6, 0, 0, ovfl };
 		state := newdecodeState(unsignedResult);
 		execDec("uint64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("uint64 a = %v not 17", data.a)
 		}
 	}
 
 	// float
 	{
 		var data struct { a float };
-		instr := &decInstr{ decFloat, 6, 0, 0 };
+		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
 		state := newdecodeState(floatResult);
 		execDec("float", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("float a = %v not 17", data.a)
 		}
 	}
 
 	// float32
 	{
 		var data struct { a float32 };
-		instr := &decInstr{ decFloat32, 6, 0, 0 };
+		instr := &decInstr{ decFloat32, 6, 0, 0, ovfl };
 		state := newdecodeState(floatResult);
 		execDec("float32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("float32 a = %v not 17", data.a)
 		}
 	}
 
 	// float64
 	{
 		var data struct { a float64 };
-		instr := &decInstr{ decFloat64, 6, 0, 0 };
+		instr := &decInstr{ decFloat64, 6, 0, 0, ovfl };
 		state := newdecodeState(floatResult);
 		execDec("float64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
-			t.Errorf("int a = %v not 17", data.a)
+			t.Errorf("float64 a = %v not 17", data.a)
 		}
 	}
 
 	// bytes == []uint8
 	{
 		var data struct { a []byte };
-		instr := &decInstr{ decUint8Array, 6, 0, 0 };
+		instr := &decInstr{ decUint8Array, 6, 0, 0, ovfl };
 		state := newdecodeState(bytesResult);
 		execDec("bytes", instr, state, t, unsafe.Pointer(&data));
 		if string(data.a) != "hello" {
@@ -515,7 +528,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	// string
 	{
 		var data struct { a string };
-		instr := &decInstr{ decString, 6, 0, 0 };
+		instr := &decInstr{ decString, 6, 0, 0, ovfl };
 		state := newdecodeState(bytesResult);
 		execDec("bytes", instr, state, t, unsafe.Pointer(&data));
 		if data.a != "hello" {
@@ -559,6 +572,157 @@ func TestEndToEnd(t *testing.T) {
 	}
 }
 
+func TestOverflow(t *testing.T) {
+	type inputT struct {
+		maxi	int64;
+		mini	int64;
+		maxu	uint64;
+		maxf	float64;
+		minf	float64;
+	}
+	var it inputT;
+	var err os.Error;
+	id := getTypeInfo(reflect.Typeof(it)).id;
+	b := new(bytes.Buffer);
+
+	// int8
+	b.Reset();
+	it = inputT {
+		maxi: math.MaxInt8 + 1,
+	};
+	type outi8 struct {
+		maxi int8;
+		mini int8;
+	}
+	var o1 outi8;
+	encode(b, it);
+	err = decode(b, id, &o1);
+	if err == nil || err.String() != `value for "maxi" out of range` {
+		t.Error("wrong overflow error for int8:", err)
+	}
+	it = inputT {
+		mini: math.MinInt8 - 1,
+	};
+	b.Reset();
+	encode(b, it);
+	err = decode(b, id, &o1);
+	if err == nil || err.String() != `value for "mini" out of range` {
+		t.Error("wrong underflow error for int8:", err)
+	}
+
+	// int16
+	b.Reset();
+	it = inputT {
+		maxi: math.MaxInt16 + 1,
+	};
+	type outi16 struct {
+		maxi int16;
+		mini int16;
+	}
+	var o2 outi16;
+	encode(b, it);
+	err = decode(b, id, &o2);
+	if err == nil || err.String() != `value for "maxi" out of range` {
+		t.Error("wrong overflow error for int16:", err)
+	}
+	it = inputT {
+		mini: math.MinInt16 - 1,
+	};
+	b.Reset();
+	encode(b, it);
+	err = decode(b, id, &o2);
+	if err == nil || err.String() != `value for "mini" out of range` {
+		t.Error("wrong underflow error for int16:", err)
+	}
+
+	// int32
+	b.Reset();
+	it = inputT {
+		maxi: math.MaxInt32 + 1,
+	};
+	type outi32 struct {
+		maxi int32;
+		mini int32;
+	}
+	var o3 outi32;
+	encode(b, it);
+	err = decode(b, id, &o3);
+	if err == nil || err.String() != `value for "maxi" out of range` {
+		t.Error("wrong overflow error for int32:", err)
+	}
+	it = inputT {
+		mini: math.MinInt32 - 1,
+	};
+	b.Reset();
+	encode(b, it);
+	err = decode(b, id, &o3);
+	if err == nil || err.String() != `value for "mini" out of range` {
+		t.Error("wrong underflow error for int32:", err)
+	}
+
+	// uint8
+	b.Reset();
+	it = inputT {
+		maxu: math.MaxUint8 + 1,
+	};
+	type outu8 struct {
+		maxu uint8;
+	}
+	var o4 outu8;
+	encode(b, it);
+	err = decode(b, id, &o4);
+	if err == nil || err.String() != `value for "maxu" out of range` {
+		t.Error("wrong overflow error for uint8:", err)
+	}
+
+	// uint16
+	b.Reset();
+	it = inputT {
+		maxu: math.MaxUint16 + 1,
+	};
+	type outu16 struct {
+		maxu uint16;
+	}
+	var o5 outu16;
+	encode(b, it);
+	err = decode(b, id, &o5);
+	if err == nil || err.String() != `value for "maxu" out of range` {
+		t.Error("wrong overflow error for uint16:", err)
+	}
+
+	// uint32
+	b.Reset();
+	it = inputT {
+		maxu: math.MaxUint32 + 1,
+	};
+	type outu32 struct {
+		maxu uint32;
+	}
+	var o6 outu32;
+	encode(b, it);
+	err = decode(b, id, &o6);
+	if err == nil || err.String() != `value for "maxu" out of range` {
+		t.Error("wrong overflow error for uint32:", err)
+	}
+
+	// float32
+	b.Reset();
+	it = inputT {
+		maxf: math.MaxFloat32 * 2,
+	};
+	type outf32 struct {
+		maxf float32;
+		minf float32;
+	}
+	var o7 outf32;
+	encode(b, it);
+	err = decode(b, id, &o7);
+	if err == nil || err.String() != `value for "maxf" out of range` {
+		t.Error("wrong overflow error for float32:", err)
+	}
+}
+
+
 func TestNesting(t *testing.T) {
 	type RT struct {
 		a string;
diff --git a/src/pkg/gob/decode.go b/src/pkg/gob/decode.go
index 7e439e8e7a..17afca6072 100644
--- a/src/pkg/gob/decode.go
+++ b/src/pkg/gob/decode.go
@@ -29,6 +29,10 @@ type decodeState struct {
 	fieldnum	int;	// the last field number read.
 }
 
+func overflow(name string) os.ErrorString {
+	return os.ErrorString(`value for "` + name + `" out of range`); 
+}
+
 // decodeUintReader reads an encoded unsigned integer from an io.Reader.
 // Used only by the Decoder to read the message length.
 func decodeUintReader(r io.Reader, oneByte []byte) (x uint64, err os.Error) {
@@ -50,6 +54,7 @@ func decodeUintReader(r io.Reader, oneByte []byte) (x uint64, err os.Error) {
 
 // decodeUint reads an encoded unsigned integer from state.r.
 // Sets state.err.  If state.err is already non-nil, it does nothing.
+// Does not check for overflow.
 func decodeUint(state *decodeState) (x uint64) {
 	if state.err != nil {
 		return
@@ -71,6 +76,7 @@ func decodeUint(state *decodeState) (x uint64) {
 
 // decodeInt reads an encoded signed integer from state.r.
 // Sets state.err.  If state.err is already non-nil, it does nothing.
+// Does not check for overflow.
 func decodeInt(state *decodeState) int64 {
 	x := decodeUint(state);
 	if state.err != nil {
@@ -91,6 +97,7 @@ type decInstr struct {
 	field		int;	// field number of the wire type
 	indir	int;	// how many pointer indirections to reach the value in the struct
 	offset	uintptr;	// offset in the structure of the field to encode
+	ovfl	os.ErrorString;	// error message for overflow/underflow (for arrays, of the elements)
 }
 
 // Since the encoder writes no zeros, if we arrive at a decoder we have
@@ -126,26 +133,6 @@ func decBool(i *decInstr, state *decodeState, p unsafe.Pointer) {
 	*(*bool)(p) = decodeInt(state) != 0;
 }
 
-func decInt(i *decInstr, state *decodeState, p unsafe.Pointer) {
-	if i.indir > 0 {
-		if *(*unsafe.Pointer)(p) == nil {
-			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
-		}
-		p = *(*unsafe.Pointer)(p);
-	}
-	*(*int)(p) = int(decodeInt(state));
-}
-
-func decUint(i *decInstr, state *decodeState, p unsafe.Pointer) {
-	if i.indir > 0 {
-		if *(*unsafe.Pointer)(p) == nil {
-			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint));
-		}
-		p = *(*unsafe.Pointer)(p);
-	}
-	*(*uint)(p) = uint(decodeUint(state));
-}
-
 func decInt8(i *decInstr, state *decodeState, p unsafe.Pointer) {
 	if i.indir > 0 {
 		if *(*unsafe.Pointer)(p) == nil {
@@ -153,7 +140,12 @@ func decInt8(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*int8)(p) = int8(decodeInt(state));
+	v := decodeInt(state);
+	if v < math.MinInt8 || math.MaxInt8 < v {
+		state.err = i.ovfl
+	} else {
+		*(*int8)(p) = int8(v)
+	}
 }
 
 func decUint8(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -163,7 +155,12 @@ func decUint8(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*uint8)(p) = uint8(decodeUint(state));
+	v := decodeUint(state);
+	if math.MaxUint8 < v {
+		state.err = i.ovfl
+	} else {
+		*(*uint8)(p) = uint8(v)
+	}
 }
 
 func decInt16(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -173,7 +170,12 @@ func decInt16(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*int16)(p) = int16(decodeInt(state));
+	v := decodeInt(state);
+	if v < math.MinInt16 || math.MaxInt16 < v {
+		state.err = i.ovfl
+	} else {
+		*(*int16)(p) = int16(v)
+	}
 }
 
 func decUint16(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -183,7 +185,12 @@ func decUint16(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*uint16)(p) = uint16(decodeUint(state));
+	v := decodeUint(state);
+	if math.MaxUint16 < v {
+		state.err = i.ovfl
+	} else {
+		*(*uint16)(p) = uint16(v)
+	}
 }
 
 func decInt32(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -193,7 +200,12 @@ func decInt32(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*int32)(p) = int32(decodeInt(state));
+	v := decodeInt(state);
+	if v < math.MinInt32 || math.MaxInt32 < v {
+		state.err = i.ovfl
+	} else {
+		*(*int32)(p) = int32(v)
+	}
 }
 
 func decUint32(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -203,7 +215,12 @@ func decUint32(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*uint32)(p) = uint32(decodeUint(state));
+	v := decodeUint(state);
+	if math.MaxUint32 < v {
+		state.err = i.ovfl
+	} else {
+		*(*uint32)(p) = uint32(v)
+	}
 }
 
 func decInt64(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -226,16 +243,6 @@ func decUint64(i *decInstr, state *decodeState, p unsafe.Pointer) {
 	*(*uint64)(p) = uint64(decodeUint(state));
 }
 
-func decUintptr(i *decInstr, state *decodeState, p unsafe.Pointer) {
-	if i.indir > 0 {
-		if *(*unsafe.Pointer)(p) == nil {
-			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uintptr));
-		}
-		p = *(*unsafe.Pointer)(p);
-	}
-	*(*uintptr)(p) = uintptr(decodeUint(state));
-}
-
 // Floating-point numbers are transmitted as uint64s holding the bits
 // of the underlying representation.  They are sent byte-reversed, with
 // the exponent end coming out first, so integer floating point numbers
@@ -251,16 +258,6 @@ func floatFromBits(u uint64) float64 {
 	return math.Float64frombits(v);
 }
 
-func decFloat(i *decInstr, state *decodeState, p unsafe.Pointer) {
-	if i.indir > 0 {
-		if *(*unsafe.Pointer)(p) == nil {
-			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(float));
-		}
-		p = *(*unsafe.Pointer)(p);
-	}
-	*(*float)(p) = float(floatFromBits(uint64(decodeUint(state))));
-}
-
 func decFloat32(i *decInstr, state *decodeState, p unsafe.Pointer) {
 	if i.indir > 0 {
 		if *(*unsafe.Pointer)(p) == nil {
@@ -268,7 +265,16 @@ func decFloat32(i *decInstr, state *decodeState, p unsafe.Pointer) {
 		}
 		p = *(*unsafe.Pointer)(p);
 	}
-	*(*float32)(p) = float32(floatFromBits(uint64(decodeUint(state))));
+	v := floatFromBits(decodeUint(state));
+	av := v;
+	if av < 0 {
+		av = -av
+	}
+	if math.MaxFloat32 < av {	// underflow is OK
+		state.err = i.ovfl
+	} else {
+		*(*float32)(p) = float32(v)
+	}
 }
 
 func decFloat64(i *decInstr, state *decodeState, p unsafe.Pointer) {
@@ -386,8 +392,8 @@ func ignoreStruct(engine *decEngine, b *bytes.Buffer) os.Error {
 	return state.err
 }
 
-func decodeArrayHelper(state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, length, elemIndir int) os.Error {
-	instr := &decInstr{elemOp, 0, elemIndir, 0};
+func decodeArrayHelper(state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, length, elemIndir int, ovfl os.ErrorString) os.Error {
+	instr := &decInstr{elemOp, 0, elemIndir, 0, ovfl};
 	for i := 0; i < length && state.err == nil; i++ {
 		up := unsafe.Pointer(p);
 		if elemIndir > 1 {
@@ -399,7 +405,7 @@ func decodeArrayHelper(state *decodeState, p uintptr, elemOp decOp, elemWid uint
 	return state.err
 }
 
-func decodeArray(atyp *reflect.ArrayType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, length, indir, elemIndir int) os.Error {
+func decodeArray(atyp *reflect.ArrayType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, length, indir, elemIndir int, ovfl os.ErrorString) os.Error {
 	if indir > 0 {
 		up := unsafe.Pointer(p);
 		if *(*unsafe.Pointer)(up) == nil {
@@ -413,11 +419,11 @@ func decodeArray(atyp *reflect.ArrayType, state *decodeState, p uintptr, elemOp
 	if n := decodeUint(state); n != uint64(length) {
 		return os.ErrorString("gob: length mismatch in decodeArray");
 	}
-	return decodeArrayHelper(state, p, elemOp, elemWid, length, elemIndir);
+	return decodeArrayHelper(state, p, elemOp, elemWid, length, elemIndir, ovfl);
 }
 
 func ignoreArrayHelper(state *decodeState, elemOp decOp, length int) os.Error {
-	instr := &decInstr{elemOp, 0, 0, 0};
+	instr := &decInstr{elemOp, 0, 0, 0, os.ErrorString("no error")};
 	for i := 0; i < length && state.err == nil; i++ {
 		elemOp(instr, state, nil);
 	}
@@ -431,7 +437,7 @@ func ignoreArray(state *decodeState, elemOp decOp, length int) os.Error {
 	return ignoreArrayHelper(state, elemOp, length);
 }
 
-func decodeSlice(atyp *reflect.SliceType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int) os.Error {
+func decodeSlice(atyp *reflect.SliceType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl os.ErrorString) os.Error {
 	length := uintptr(decodeUint(state));
 	if indir > 0 {
 		up := unsafe.Pointer(p);
@@ -448,7 +454,7 @@ func decodeSlice(atyp *reflect.SliceType, state *decodeState, p uintptr, elemOp
 	hdrp.Data = uintptr(unsafe.Pointer(&data[0]));
 	hdrp.Len = int(length);
 	hdrp.Cap = int(length);
-	return decodeArrayHelper(state, hdrp.Data, elemOp, elemWid, int(length), elemIndir);
+	return decodeArrayHelper(state, hdrp.Data, elemOp, elemWid, int(length), elemIndir, ovfl);
 }
 
 func ignoreSlice(state *decodeState, elemOp decOp) os.Error {
@@ -456,22 +462,18 @@ func ignoreSlice(state *decodeState, elemOp decOp) os.Error {
 }
 
 var decOpMap = map[reflect.Type] decOp {
-	reflect.Typeof((*reflect.BoolType)(nil)): decBool,
-	reflect.Typeof((*reflect.IntType)(nil)): decInt,
-	reflect.Typeof((*reflect.Int8Type)(nil)): decInt8,
-	reflect.Typeof((*reflect.Int16Type)(nil)): decInt16,
-	reflect.Typeof((*reflect.Int32Type)(nil)): decInt32,
-	reflect.Typeof((*reflect.Int64Type)(nil)): decInt64,
-	reflect.Typeof((*reflect.UintType)(nil)): decUint,
-	reflect.Typeof((*reflect.Uint8Type)(nil)): decUint8,
-	reflect.Typeof((*reflect.Uint16Type)(nil)): decUint16,
-	reflect.Typeof((*reflect.Uint32Type)(nil)): decUint32,
-	reflect.Typeof((*reflect.Uint64Type)(nil)): decUint64,
-	reflect.Typeof((*reflect.UintptrType)(nil)): decUintptr,
-	reflect.Typeof((*reflect.FloatType)(nil)): decFloat,
-	reflect.Typeof((*reflect.Float32Type)(nil)): decFloat32,
-	reflect.Typeof((*reflect.Float64Type)(nil)): decFloat64,
-	reflect.Typeof((*reflect.StringType)(nil)): decString,
+	valueKind(false): decBool,
+	valueKind(int8(0)): decInt8,
+	valueKind(int16(0)): decInt16,
+	valueKind(int32(0)): decInt32,
+	valueKind(int64(0)): decInt64,
+	valueKind(uint8(0)): decUint8,
+	valueKind(uint16(0)): decUint16,
+	valueKind(uint32(0)): decUint32,
+	valueKind(uint64(0)): decUint64,
+	valueKind(float32(0)): decFloat32,
+	valueKind(float64(0)): decFloat64,
+	valueKind("x"): decString,
 }
 
 var decIgnoreOpMap = map[typeId] decOp {
@@ -488,34 +490,38 @@ func getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, err os.Error)
 
 // Return the decoding op for the base type under rt and
 // the indirection count to reach it.
-func decOpFor(wireId typeId, rt reflect.Type) (decOp, int, os.Error) {
+func decOpFor(wireId typeId, rt reflect.Type, name string) (decOp, int, os.Error) {
 	typ, indir := indirect(rt);
 	op, ok := decOpMap[reflect.Typeof(typ)];
 	if !ok {
 		// Special cases
 		switch t := typ.(type) {
 		case *reflect.SliceType:
+			name = "element of " + name;
 			if _, ok := t.Elem().(*reflect.Uint8Type); ok {
 				op = decUint8Array;
 				break;
 			}
 			elemId := wireId.gobType().(*sliceType).Elem;
-			elemOp, elemIndir, err := decOpFor(elemId, t.Elem());
+			elemOp, elemIndir, err := decOpFor(elemId, t.Elem(), name);
 			if err != nil {
 				return nil, 0, err
 			}
+			ovfl := overflow(name);
 			op = func(i *decInstr, state *decodeState, p unsafe.Pointer) {
-				state.err = decodeSlice(t, state, uintptr(p), elemOp, t.Elem().Size(), i.indir, elemIndir);
+				state.err = decodeSlice(t, state, uintptr(p), elemOp, t.Elem().Size(), i.indir, elemIndir, ovfl);
 			};
 
 		case *reflect.ArrayType:
+			name = "element of " + name;
 			elemId := wireId.gobType().(*arrayType).Elem;
-			elemOp, elemIndir, err := decOpFor(elemId, t.Elem());
+			elemOp, elemIndir, err := decOpFor(elemId, t.Elem(), name);
 			if err != nil {
 				return nil, 0, err
 			}
+			ovfl := overflow(name);
 			op = func(i *decInstr, state *decodeState, p unsafe.Pointer) {
-				state.err = decodeArray(t, state, uintptr(p), elemOp, t.Elem().Size(), t.Len(), i.indir, elemIndir);
+				state.err = decodeArray(t, state, uintptr(p), elemOp, t.Elem().Size(), t.Len(), i.indir, elemIndir, ovfl);
 			};
 
 		case *reflect.StructType:
@@ -658,23 +664,25 @@ func compileDec(wireId typeId, rt reflect.Type) (engine *decEngine, err os.Error
 		wireField := wireStruct.field[fieldnum];
 		// Find the field of the local type with the same name.
 		localField, present := srt.FieldByName(wireField.name);
+		ovfl := overflow(wireField.name);
 		// TODO(r): anonymous names
 		if !present || localField.Anonymous {
 			op, err := decIgnoreOpFor(wireField.id);
 			if err != nil {
 				return nil, err
 			}
-			engine.instr[fieldnum] = decInstr{op, fieldnum, 0, 0};
+			engine.instr[fieldnum] = decInstr{op, fieldnum, 0, 0, ovfl};
 			continue;
 		}
 		if !compatibleType(localField.Type, wireField.id) {
-			return nil, os.ErrorString("gob: wrong type for field " + wireField.name + " in type " + wireId.Name());
+			details := " (" + wireField.id.String() + " incompatible with " + localField.Type.String() + ") in type " + wireId.Name();
+			return nil, os.ErrorString("gob: wrong type for field " + wireField.name + details);
 		}
-		op, indir, err := decOpFor(wireField.id, localField.Type);
+		op, indir, err := decOpFor(wireField.id, localField.Type, localField.Name);
 		if err != nil {
 			return nil, err
 		}
-		engine.instr[fieldnum] = decInstr{op, fieldnum, indir, uintptr(localField.Offset)};
+		engine.instr[fieldnum] = decInstr{op, fieldnum, indir, uintptr(localField.Offset), ovfl};
 		engine.numInstr++;
 	}
 	return;
@@ -746,3 +754,44 @@ func decode(b *bytes.Buffer, wireId typeId, e interface{}) os.Error {
 	}
 	return decodeStruct(engine, rt.(*reflect.StructType), b, uintptr(v.Addr()), 0);
 }
+
+func init() {
+	// We assume that the size of float is sufficient to tell us whether it is
+	// equivalent to float32 or to float64.   This is very unlikely to be wrong.
+	var op decOp;
+	switch unsafe.Sizeof(float(0)) {
+	case unsafe.Sizeof(float32(0)):
+		op = decFloat32;
+	case unsafe.Sizeof(float64(0)):
+		op = decFloat64;
+	default:
+		panic("gob: unknown size of float", unsafe.Sizeof(float(0)));
+	}
+	decOpMap[valueKind(float(0))] = op;
+
+	// A similar assumption about int and uint.  Also assume int and uint have the same size.
+	var uop decOp;
+	switch unsafe.Sizeof(int(0)) {
+	case unsafe.Sizeof(int32(0)):
+		op = decInt32;
+		uop = decUint32;
+	case unsafe.Sizeof(int64(0)):
+		op = decInt64;
+		uop = decUint64;
+	default:
+		panic("gob: unknown size of int/uint", unsafe.Sizeof(int(0)));
+	}
+	decOpMap[valueKind(int(0))] = op;
+	decOpMap[valueKind(uint(0))] = uop;
+
+	// Finally uintptr
+	switch unsafe.Sizeof(uintptr(0)) {
+	case unsafe.Sizeof(uint32(0)):
+		uop = decUint32;
+	case unsafe.Sizeof(uint64(0)):
+		uop = decUint64;
+	default:
+		panic("gob: unknown size of uintptr", unsafe.Sizeof(uintptr(0)));
+	}
+	decOpMap[valueKind(uintptr(0))] = uop;
+}
diff --git a/src/pkg/gob/encode.go b/src/pkg/gob/encode.go
index bfa2d69050..0589f38632 100644
--- a/src/pkg/gob/encode.go
+++ b/src/pkg/gob/encode.go
@@ -309,22 +309,22 @@ func encodeArray(b *bytes.Buffer, p uintptr, op encOp, elemWid uintptr, length i
 }
 
 var encOpMap = map[reflect.Type] encOp {
-	reflect.Typeof((*reflect.BoolType)(nil)): encBool,
-	reflect.Typeof((*reflect.IntType)(nil)): encInt,
-	reflect.Typeof((*reflect.Int8Type)(nil)): encInt8,
-	reflect.Typeof((*reflect.Int16Type)(nil)): encInt16,
-	reflect.Typeof((*reflect.Int32Type)(nil)): encInt32,
-	reflect.Typeof((*reflect.Int64Type)(nil)): encInt64,
-	reflect.Typeof((*reflect.UintType)(nil)): encUint,
-	reflect.Typeof((*reflect.Uint8Type)(nil)): encUint8,
-	reflect.Typeof((*reflect.Uint16Type)(nil)): encUint16,
-	reflect.Typeof((*reflect.Uint32Type)(nil)): encUint32,
-	reflect.Typeof((*reflect.Uint64Type)(nil)): encUint64,
-	reflect.Typeof((*reflect.UintptrType)(nil)): encUintptr,
-	reflect.Typeof((*reflect.FloatType)(nil)): encFloat,
-	reflect.Typeof((*reflect.Float32Type)(nil)): encFloat32,
-	reflect.Typeof((*reflect.Float64Type)(nil)): encFloat64,
-	reflect.Typeof((*reflect.StringType)(nil)): encString,
+	valueKind(false): encBool,
+	valueKind(int(0)): encInt,
+	valueKind(int8(0)): encInt8,
+	valueKind(int16(0)): encInt16,
+	valueKind(int32(0)): encInt32,
+	valueKind(int64(0)): encInt64,
+	valueKind(uint(0)): encUint,
+	valueKind(uint8(0)): encUint8,
+	valueKind(uint16(0)): encUint16,
+	valueKind(uint32(0)): encUint32,
+	valueKind(uint64(0)): encUint64,
+	valueKind(uintptr(0)): encUintptr,
+	valueKind(float(0)): encFloat,
+	valueKind(float32(0)): encFloat32,
+	valueKind(float64(0)): encFloat64,
+	valueKind("x"): encString,
 }
 
 func getEncEngine(rt reflect.Type) *encEngine
diff --git a/src/pkg/gob/encoder.go b/src/pkg/gob/encoder.go
index 1182a70c43..1a401eb1c5 100644
--- a/src/pkg/gob/encoder.go
+++ b/src/pkg/gob/encoder.go
@@ -66,7 +66,7 @@
 	unsigned integers may be received into any unsigned integer variable; and floating
 	point values may be received into any floating point variable.  However,
 	the destination variable must be able to represent the value or the decode
-	operation will fail. (TODO(r): enforce this.)
+	operation will fail.
 
 	Structs, arrays and slices are also supported.  Strings and arrays of bytes are
 	supported with a special, efficient representation (see below).
diff --git a/src/pkg/gob/type.go b/src/pkg/gob/type.go
index 68d047ffd3..f54746c323 100644
--- a/src/pkg/gob/type.go
+++ b/src/pkg/gob/type.go
@@ -13,6 +13,30 @@ import (
 	"unicode";
 )
 
+type kind reflect.Type
+
+// Reflection types are themselves interface values holding structs
+// describing the type.  Each type has a different struct so that struct can
+// be the kind.  For example, if typ is the reflect type for an int8, typ is
+// a pointer to a reflect.Int8Type struct; if typ is the reflect type for a
+// function, typ is a pointer to a reflect.FuncType struct; we use the type
+// of that pointer as the kind.
+
+// typeKind returns a reflect.Type representing typ's kind.  The kind is the
+// general kind of type:
+//	int8, int16, int, uint, float, func, chan, struct, and so on.
+// That is, all struct types have the same kind, all func types have the same
+// kind, all int8 types have the same kind, and so on.
+func typeKind(typ reflect.Type) kind {
+	return kind(reflect.Typeof(typ));
+}
+
+// valueKind returns the kind of the value type
+// stored inside the interface v.
+func valueKind(v interface{}) reflect.Type {
+	return typeKind(reflect.Typeof(v));
+}
+
 // A typeId represents a gob Type as an integer that can be passed on the wire.
 // Internally, typeIds are used as keys to a map to recover the underlying type info.
 type typeId int32