cgen: fix selector accessing field (#17567)

vlib/v/gen/c/cgen.v

@@ -3461,7 +3461,12 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 			g.write('*(')
 		}
 		g.expr(node.expr)
-		g.write('.${field_name}')
+		if node.expr_type.is_ptr() {
+			g.write('->')
+		} else {
+			g.write('.')
+		}
+		g.write(field_name)
 		if is_ptr {
 			g.write(')')
 		}

vlib/v/tests/option_selector_test.v

@@ -0,0 +1,27 @@
+module main
+
+struct JsonParser {
+mut:
+	js        string
+	tokens    []string
+	pos       int
+	tok_next  int
+	tok_super ?int
+}
+
+fn new_parser(qty int) JsonParser {
+	return JsonParser{
+		tokens: []string{cap: qty}
+	}
+}
+
+fn (mut p JsonParser) parse() int {
+	j := p.tok_super or { 999 }
+	return j
+}
+
+fn test_main() {
+	mut x := new_parser(100)
+	i := x.parse()
+	assert i == 999
+}