From d5d392a2e592b29d508b01d0cde25af6c85625a1 Mon Sep 17 00:00:00 2001
From: larpon <768942+larpon@users.noreply.github.com>
Date: Tue, 26 Aug 2025 12:22:12 +0200
Subject: [PATCH] flag: fix tail/single bool flag error condition logic in
 `flag_to.v` (fix #25166) (#25172)

---
 vlib/flag/flag_to.v                |  4 ++-
 vlib/flag/flag_to_tail_bool_test.v | 42 ++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 vlib/flag/flag_to_tail_bool_test.v

diff --git a/vlib/flag/flag_to.v b/vlib/flag/flag_to.v
index 8f46fd20c2..876d65d995 100644
--- a/vlib/flag/flag_to.v
+++ b/vlib/flag/flag_to.v
@@ -934,7 +934,9 @@ pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 						.f64()
 				} $else $if field.typ is bool {
 					if arg := f.arg {
-						return error('can not assign `${arg}` to bool field `${field.name}`')
+						if arg != '' {
+							return error('can not assign `${arg}` to bool field `${field.name}`')
+						}
 					}
 					result.$(field.name) = !the_default.$(field.name)
 				} $else $if field.typ is string {
diff --git a/vlib/flag/flag_to_tail_bool_test.v b/vlib/flag/flag_to_tail_bool_test.v
new file mode 100644
index 0000000000..a67e923f44
--- /dev/null
+++ b/vlib/flag/flag_to_tail_bool_test.v
@@ -0,0 +1,42 @@
+import flag
+
+const args_bool_short = ['some.exe', '-h']
+const args_bool_long = ['some.exe', '-help']
+
+struct CliOptions {
+	show_help bool @[long: 'help'; short: h]
+}
+
+fn test_short_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_short,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+}
+
+fn test_long_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_long,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+}