parser: fix forwarding vargs & multiple call varg len fix

vlib/compiler/tests/fn_variadic_test.v

@@ -2,6 +2,7 @@ struct VaTestGroup {
 	name string
 }
 
+// basic
 fn variadic_test(name string, groups ...VaTestGroup) {
 	assert groups.len == 2
 	assert groups[0].name == 'users' 
@@ -14,6 +15,7 @@ fn test_fn_variadic() {
 	variadic_test('joe', group1, group2)
 }
 
+// generic
 fn variadic_test_generic<T>(a int, b ...T) T {
 	b1 := b[0]
 	b2 := b[1]
@@ -23,3 +25,19 @@ fn variadic_test_generic<T>(a int, b ...T) T {
 fn test_fn_variadic_generic() {
 	assert variadic_test_generic(111, 'hello', 'v') == '111 hello v'
 }
+
+// forwarding
+fn variadic_forward_a(a ...string) string {
+	return variadic_forward_b(a)
+}
+
+fn variadic_forward_b(a ...string) string {
+	a0 := a[0]
+	a1 := a[1]
+	a2 := a[2]
+	return '$a0$a1$a2'
+}
+
+fn test_fn_variadic_forward() {
+	assert variadic_forward_a('a', 'b', 'c') == 'abc'
+}