gen: fix mut sumtype arguments for fields (#13205)

vlib/v/gen/c/fn.v

@@ -1647,8 +1647,8 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 				}
 				return
 			} else if arg_sym.kind == .sum_type && exp_sym.kind == .sum_type
-				&& arg.expr is ast.Ident {
-				g.write('&')
+				&& (arg.expr is ast.Ident || arg.expr is ast.SelectorExpr) {
+				g.write('&/*sum*/')
 				g.expr(arg.expr)
 				return
 			}

vlib/v/tests/fn_call_mut_sumtype_args_test.v

@@ -1,11 +1,18 @@
-type Sum = Struct1 | int
+type Sum1 = Struct1 | int
 
 struct Struct1 {
 mut:
 	value int
 }
 
-fn update_sum(mut sum Sum) {
+type Sum2 = Struct1 | Struct2 | int
+
+struct Struct2 {
+mut:
+	sum Sum2
+}
+
+fn update_sum_1(mut sum Sum1) {
 	match mut sum {
 		Struct1 {
 			sum.value = 42
@@ -14,12 +21,24 @@ fn update_sum(mut sum Sum) {
 	}
 }
 
+fn update_sum_2(mut sum Sum2) {
+	match mut sum {
+		Struct1 {
+			sum.value = 42
+		}
+		Struct2 {
+			update_sum_2(mut sum.sum)
+		}
+		else {}
+	}
+}
+
 fn test_fn_call_mut_sumtype_args() {
-	mut s := Sum(Struct1{
+	mut s := Sum1(Struct1{
 		value: 6
 	})
 
-	update_sum(mut s)
+	update_sum_1(mut s)
 
 	if mut s is Struct1 {
 		println(s.value)
@@ -28,3 +47,24 @@ fn test_fn_call_mut_sumtype_args() {
 		assert false
 	}
 }
+
+fn test_fn_call_mut_sumtype_args_field() {
+	mut s := Sum2(Struct2{
+		sum: Sum2(Struct1{
+			value: 6
+		})
+	})
+
+	update_sum_2(mut s)
+
+	if mut s is Struct2 {
+		if mut s.sum is Struct1 {
+			println(s.sum.value)
+			assert s.sum.value == 42
+		} else {
+			assert false
+		}
+	} else {
+		assert false
+	}
+}