flag: improve the fix for #25166 tail/single bool flag error in flag_to.v (#25189)

vlib/flag/flag_to.v

@@ -934,10 +934,8 @@ pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 						.f64()
 				} $else $if field.typ is bool {
 					if arg := f.arg {
-						if arg != '' {
 						return error('can not assign `${arg}` to bool field `${field.name}`')
 					}
-					}
 					result.$(field.name) = !the_default.$(field.name)
 				} $else $if field.typ is string {
 					trace_dbg_println('${@FN}: assigning (string) ${struct_name}.${field.name} = ${f.arg or {
@@ -1037,7 +1035,7 @@ fn (mut fm FlagMapper) map_v(flag_ctx FlagContext, field StructField) !bool {
 	next := flag_ctx.next
 
 	if field.hints.has(.is_bool) {
-		if flag_name == field.match_name {
+		if flag_name == field.match_name || flag_name == field.short {
 			arg := if flag_raw.contains('=') { flag_raw.all_after('=') } else { '' }
 			if arg != '' {
 				return error('flag `${flag_raw}` can not be assigned to bool field "${field.name}"')

vlib/flag/flag_to_tail_bool_test.v

@@ -1,14 +1,22 @@
 import flag
 
-const args_bool_short = ['some.exe', '-h']
-const args_bool_long = ['some.exe', '-help']
+const args_bool_short_tail = ['some.exe', '-h']
+const args_bool_long_tail = ['some.exe', '-help']
+
+const args_bool_short_mixed = ['some.exe', '-h', '-long', 'val']
+const args_bool_long_mixed = ['some.exe', '-help', '-long', 'val']
 
 struct CliOptions {
 	show_help bool @[long: 'help'; short: h]
 }
 
-fn test_short_tail_bool() {
-	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_short,
+struct CliOptions2 {
+	show_help bool @[long: 'help'; short: h]
+	long      string
+}
+
+fn test_v_style_short_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_short_tail,
 		skip:  1
 		style: .v
 		mode:  .relaxed
@@ -24,8 +32,8 @@ fn test_short_tail_bool() {
 	}
 }
 
-fn test_long_tail_bool() {
-	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_long,
+fn test_v_style_long_tail_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions](args_bool_long_tail,
 		skip:  1
 		style: .v
 		mode:  .relaxed
@@ -40,3 +48,39 @@ fn test_long_tail_bool() {
 		assert false
 	}
 }
+
+fn test_v_style_short_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions2](args_bool_short_mixed,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+	assert cli_options.long == 'val'
+}
+
+fn test_v_style_long_bool() {
+	cli_options, unmatched := flag.to_struct[CliOptions2](args_bool_long_mixed,
+		skip:  1
+		style: .v
+		mode:  .relaxed
+	)!
+
+	if unmatched.len > 0 {
+		assert false
+	}
+	if cli_options.show_help {
+		assert true
+	} else {
+		assert false
+	}
+	assert cli_options.long == 'val'
+}