reflect: fix Slice cap

R=golang-dev, dsymonds, r, rsc
CC=golang-dev
https://golang.org/cl/5483044

src/pkg/reflect/all_test.go

@@ -1557,14 +1557,26 @@ func TestSmallNegativeInt(t *testing.T) {
 func TestSlice(t *testing.T) {
 	xs := []int{1, 2, 3, 4, 5, 6, 7, 8}
 	v := ValueOf(xs).Slice(3, 5).Interface().([]int)
-	if len(v) != 2 || v[0] != 4 || v[1] != 5 {
-		t.Errorf("xs.Slice(3, 5) = %v", v)
+	if len(v) != 2 {
+		t.Errorf("len(xs.Slice(3, 5)) = %d", len(v))
+	}
+	if cap(v) != 5 {
+		t.Errorf("cap(xs.Slice(3, 5)) = %d", cap(v))
+	}
+	if !DeepEqual(v[0:5], xs[3:]) {
+		t.Errorf("xs.Slice(3, 5)[0:5] = %v", v[0:5])
 	}
 
-	xa := [7]int{10, 20, 30, 40, 50, 60, 70}
+	xa := [8]int{10, 20, 30, 40, 50, 60, 70, 80}
 	v = ValueOf(&xa).Elem().Slice(2, 5).Interface().([]int)
-	if len(v) != 3 || v[0] != 30 || v[1] != 40 || v[2] != 50 {
-		t.Errorf("xa.Slice(2, 5) = %v", v)
+	if len(v) != 3 {
+		t.Errorf("len(xa.Slice(2, 5)) = %d", len(v))
+	}
+	if cap(v) != 6 {
+		t.Errorf("cap(xa.Slice(2, 5)) = %d", cap(v))
+	}
+	if !DeepEqual(v[0:6], xa[2:]) {
+		t.Errorf("xs.Slice(2, 5)[0:6] = %v", v[0:6])
 	}
 }
 

src/pkg/reflect/value.go

@@ -1356,7 +1356,7 @@ func (v Value) Slice(beg, end int) Value {
 	s := (*SliceHeader)(unsafe.Pointer(&x))
 	s.Data = uintptr(base) + uintptr(beg)*toCommonType(typ.elem).Size()
 	s.Len = end - beg
-	s.Cap = end - beg
+	s.Cap = cap - beg
 
 	fl := v.flag&flagRO | flagIndir | flag(Slice)<<flagKindShift
 	return Value{typ.common(), unsafe.Pointer(&x), fl}