fix: automatic type conversion for untyped arg of append (#177)

_test/a21.go

@@ -0,0 +1,14 @@
+package main
+
+import "fmt"
+
+func main() {
+	a := []byte("hello")
+	fmt.Println(a)
+	a = append(a, '=')
+	fmt.Println(a)
+}
+
+// Output:
+// [104 101 108 108 111]
+// [104 101 108 108 111 61]

interp/run.go

@@ -1406,7 +1406,11 @@ func _append(n *Node) {
 		l := len(args)
 		values := make([]func(*Frame) reflect.Value, l)
 		for i, arg := range args {
-			values[i] = genValue(arg)
+			if arg.typ.untyped {
+				values[i] = genValueAs(arg, n.child[1].typ.TypeOf().Elem())
+			} else {
+				values[i] = genValue(arg)
+			}
 		}
 
 		n.exec = func(f *Frame) Builtin {
@@ -1419,6 +1423,9 @@ func _append(n *Node) {
 		}
 	} else {
 		value0 := genValue(n.child[2])
+		if n.child[2].typ.untyped {
+			value0 = genValueAs(n.child[2], n.child[1].typ.TypeOf().Elem())
+		}
 
 		n.exec = func(f *Frame) Builtin {
 			dest(f).Set(reflect.Append(value(f), value0(f)))

interp/value.go

@@ -39,6 +39,13 @@ func genValueRecv(n *Node) func(*Frame) reflect.Value {
 	}
 }
 
+func genValueAs(n *Node, t reflect.Type) func(*Frame) reflect.Value {
+	v := genValue(n)
+	return func(f *Frame) reflect.Value {
+		return v(f).Convert(t)
+	}
+}
+
 func genValue(n *Node) func(*Frame) reflect.Value {
 	switch n.kind {
 	case BasicLit, FuncDecl: