fix: append from / to runtime defined array

Check first for runtime defined array (typ.cat of valueT)
to avoid checking inexisting properties (an panic), when
deciding to use appendSlice or not.

Fixes #880.

_test/append1.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+)
+
+func main() {
+	s := bufio.NewScanner(bytes.NewReader([]byte("Hello\nTest\nLine3")))
+	s.Scan()
+	println(string(append(s.Bytes(), []byte(" World")...)))
+}
+
+// Output:
+// Hello World

_test/append2.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+)
+
+func main() {
+	s := bufio.NewScanner(bytes.NewReader([]byte("Hello\nTest\nLine3")))
+	s.Scan()
+	println(string(append(s.Bytes(), " World"...)))
+}
+
+// Output:
+// Hello World

_test/issue-880.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+)
+
+func main() {
+	var buf1 = make([]byte, 1024)
+	var buf2 []byte
+	buf1 = []byte("Hallo\nTest\nLine3")
+
+	s := bufio.NewScanner(bytes.NewReader(buf1))
+	for s.Scan() {
+		buf2 = append(buf2, append(s.Bytes(), []byte("\n")...)...)
+	}
+	print(string(buf2))
+}
+
+// Output:
+// Hallo
+// Test
+// Line3

interp/run.go

@@ -2430,11 +2430,16 @@ func appendSlice(n *node) {
 }
 
 func _append(n *node) {
-	if c1, c2 := n.child[1], n.child[2]; len(n.child) == 3 && c2.typ.cat == arrayT && c2.typ.val.id() == n.typ.val.id() ||
-		isByteArray(c1.typ.TypeOf()) && isString(c2.typ.TypeOf()) {
-		appendSlice(n)
-		return
+	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.val.id() == n.typ.val.id() ||
+			isByteArray(c1.typ.TypeOf()) && isString(c2.typ.TypeOf()) {
+			appendSlice(n)
+			return
+		}
 	}
+
 	dest := genValueOutput(n, n.typ.rtype)
 	value := genValue(n.child[1])
 	next := getExec(n.tnext)