interp: fix append a slice on binary slice

Fixes #1128.

_test/append3.go

@@ -0,0 +1,14 @@
+package main
+
+import "fmt"
+
+func main() {
+	a := []int{1, 2}
+	b := [2]int{3, 4}
+	fmt.Println(append(a, b[:]...))
+	fmt.Println(append(a, []int{5, 6}...))
+}
+
+// Output:
+// [1 2 3 4]
+// [1 2 5 6]

_test/issue-1128.go

@@ -0,0 +1,10 @@
+package main
+
+import "net"
+
+func main() {
+	c := append(net.Buffers{}, []byte{})
+	println(len(c))
+}
+
+// Output: 1

interp/run.go

@@ -2917,7 +2917,7 @@ func _append(n *node) {
 	if len(n.child) == 3 {
 		c1, c2 := n.child[1], n.child[2]
 		if (c1.typ.cat == valueT || c2.typ.cat == valueT) && c1.typ.rtype == c2.typ.rtype ||
-			(c2.typ.cat == arrayT || c2.typ.cat == sliceT || c2.typ.cat == variadicT) && c2.typ.val.id() == n.typ.val.id() ||
+			isArray(c2.typ) && c2.typ.elem().id() == n.typ.elem().id() ||
 			isByteArray(c1.typ.TypeOf()) && isString(c2.typ.TypeOf()) {
 			appendSlice(n)
 			return
@@ -2957,13 +2957,13 @@ func _append(n *node) {
 		}
 	} else {
 		var value0 func(*frame) reflect.Value
-		switch {
-		case isEmptyInterface(n.typ.val):
+		switch elem := n.typ.elem(); {
+		case isEmptyInterface(elem):
 			value0 = genValue(n.child[2])
-		case isInterfaceSrc(n.typ.val):
+		case isInterfaceSrc(elem):
 			value0 = genValueInterface(n.child[2])
-		case isRecursiveType(n.typ.val, n.typ.val.rtype):
-			value0 = genValueRecursiveInterface(n.child[2], n.typ.val.rtype)
+		case isRecursiveType(elem, elem.rtype):
+			value0 = genValueRecursiveInterface(n.child[2], elem.rtype)
 		case n.child[2].typ.untyped:
 			value0 = genValueAs(n.child[2], n.child[1].typ.TypeOf().Elem())
 		default:

interp/type.go

@@ -1601,6 +1601,13 @@ func (t *itype) hasNil() bool {
 	return false
 }
 
+func (t *itype) elem() *itype {
+	if t.cat == valueT {
+		return &itype{cat: valueT, rtype: t.rtype.Elem()}
+	}
+	return t.val
+}
+
 func copyDefined(m map[string]*itype) map[string]*itype {
 	n := make(map[string]*itype, len(m))
 	for k, v := range m {