cgen: fix code generation wrong, when '?foo.array or {}' as a 'for-in' condition (fix #20528) (#20542)

vlib/v/gen/c/for.v

@@ -235,7 +235,7 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 		}
 		mut cond_var := ''
 		if (node.cond is ast.Ident && !node.cond_type.has_flag(.option))
-			|| node.cond is ast.SelectorExpr {
+			|| (node.cond is ast.SelectorExpr && node.cond.or_block.kind == .absent) {
 			cond_var = g.expr_string(node.cond)
 		} else {
 			cond_var = g.new_tmp_var()

vlib/v/tests/for_in_option_test.v

@@ -6,3 +6,23 @@ fn test_for_in_option() {
 	}
 	assert true
 }
+
+// for issue 20528
+// phenomenon: when the cond expr is SelectorExpr and the type is an array, cgen fails.
+struct Foo {
+	data ?[]int
+}
+
+fn (f Foo) get_first() ?int {
+	for d in f.data or { return none } {
+		return d
+	}
+	return none
+}
+
+fn test_cond_is_selector_array_and_with_or_block() {
+	foo := Foo{
+		data: [1, 2, 3]
+	}
+	assert foo.get_first()? == 1
+}