tests: add as_cast_already_smartcast_sumtype_test.v (#10517)

vlib/v/tests/as_cast_already_smartcast_sumtype_test.v

@@ -0,0 +1,30 @@
+struct S1 {
+	s1 string = 'abc'
+}
+
+struct Empty {
+}
+
+type Sum = Empty | S1
+
+fn test_as_cast_already_smartcast_sumtype() {
+	a := Sum(S1{})
+	if a is S1 {
+		println('if expr: $a.s1')
+		assert a.s1 == 'abc'
+		v1 := a as S1
+		println('if expr (as cast): $v1.s1')
+		assert v1.s1 == 'abc'
+	}
+
+	match a {
+		S1 {
+			println('match expr: $a.s1')
+			assert a.s1 == 'abc'
+			v1 := a as S1
+			println('match expr (as cast): $v1.s1')
+			assert v1.s1 == 'abc'
+		}
+		else {}
+	}
+}