flag: fix assigning to @[tail] field when no fields has been matche…

…d yet in `flag.parse[T]()` (#22043)

vlib/flag/flag_to.v

@@ -524,8 +524,9 @@ pub fn (mut fm FlagMapper) parse[T]() ! {
 			}
 		}
 
-		// Extract any tail according to config
+		// Extract any tail(s) according to config
 		if pos >= pos_last_flag + 1 {
+			trace_dbg_println('${@FN}: (tail) looking for tail match for position "${pos}"...')
 			pos_is_handled = pos in fm.handled_pos
 			if pos_is_handled {
 				trace_dbg_println('${@FN}: (tail) skipping position "${pos}". Already handled')
@@ -542,27 +543,27 @@ pub fn (mut fm FlagMapper) parse[T]() ! {
 					continue
 				}
 				if field.hints.has(.has_tail) {
-					if last_handled_pos := fm.handled_pos[fm.handled_pos.len - 1] {
-						trace_println('${@FN}: (tail) flag `${arg}` last_handled_pos: ${last_handled_pos} pos: ${pos}')
-						if pos == last_handled_pos + 1 {
-							if field.hints.has(.is_array) {
-								fm.array_field_map_flag[field.name] << FlagData{
-									raw:        arg
-									field_name: field.name
-									arg:        ?string(arg) // .arg is used when assigning at comptime to []XYZ
-									pos:        pos
-								}
-							} else {
-								fm.field_map_flag[field.name] = FlagData{
-									raw:        arg
-									field_name: field.name
-									arg:        ?string(arg)
-									pos:        pos
-								}
+					trace_dbg_println('${@FN}: (tail) field "${field.name}" has a tail attribute. fm.handled_pos.len: ${fm.handled_pos.len}')
+					last_handled_pos := fm.handled_pos[fm.handled_pos.len - 1] or { 0 }
+					trace_println('${@FN}: (tail) flag `${arg}` last_handled_pos: ${last_handled_pos} pos: ${pos}')
+					if pos == last_handled_pos + 1 || pos == pos_last_flag + 1 {
+						if field.hints.has(.is_array) {
+							fm.array_field_map_flag[field.name] << FlagData{
+								raw:        arg
+								field_name: field.name
+								arg:        ?string(arg) // .arg is used when assigning at comptime to []XYZ
+								pos:        pos
+							}
+						} else {
+							fm.field_map_flag[field.name] = FlagData{
+								raw:        arg
+								field_name: field.name
+								arg:        ?string(arg)
+								pos:        pos
 							}
-							fm.handled_pos << pos
-							continue
 						}
+						fm.handled_pos << pos
+						continue
 					}
 				}
 			}
@@ -900,6 +901,10 @@ pub fn (fm FlagMapper) to_struct[T](defaults ?T) !T {
 					}
 					result.$(field.name) = true
 				} $else $if field.typ is string {
+					trace_dbg_println('${@FN}: assigning (string) ${struct_name}.${field.name} = ${f.arg or {
+						'ERROR'
+					}
+						.str()}')
 					result.$(field.name) = f.arg or {
 						return error('failed appending ${f.raw} to ${field.name}')
 					}

vlib/flag/flag_to_tail_test.v

@@ -0,0 +1,62 @@
+import flag
+
+const posix_and_gnu_args_no_tail = ['-f']
+
+const posix_and_gnu_args_tail = ['-f', 'tail']
+const posix_and_gnu_args_tails = ['-f', 'tail0', 'tail1', 'tail2']
+
+const skip_posix_and_gnu_args_tail = ['skipme', '-f', 'tail']
+const skip_posix_and_gnu_args_tails = ['skipme', '-f', 'tail0', 'tail1', 'tail2']
+
+struct ConfigTail {
+	mix  bool
+	path string @[tail]
+}
+
+struct ConfigTails {
+	mix   bool
+	paths []string @[tail]
+}
+
+fn test_flag_tail() {
+	// Test `@[tail]` edge-cases
+	config1, no_matches1 := flag.to_struct[ConfigTail](posix_and_gnu_args_tail)!
+	assert config1.mix == false
+	assert config1.path == 'tail'
+	assert no_matches1 == ['-f']
+
+	config2, no_matches2 := flag.to_struct[ConfigTails](posix_and_gnu_args_tails)!
+	assert config2.mix == false
+	assert config2.paths == ['tail0', 'tail1', 'tail2']
+	assert no_matches2 == ['-f']
+
+	config3, no_matches3 := flag.to_struct[ConfigTail](skip_posix_and_gnu_args_tail, skip: 1)!
+	assert config3.mix == false
+	assert config3.path == 'tail'
+	assert no_matches3 == ['-f']
+
+	config4, no_matches4 := flag.to_struct[ConfigTails](skip_posix_and_gnu_args_tails, skip: 1)!
+	assert config4.mix == false
+	assert config4.paths == ['tail0', 'tail1', 'tail2']
+	assert no_matches4 == ['-f']
+
+	config5, no_matches5 := flag.to_struct[ConfigTail](posix_and_gnu_args_tail, skip: 1)!
+	assert config5.mix == false
+	assert config5.path == 'tail'
+	assert no_matches5 == []
+
+	config6, no_matches6 := flag.to_struct[ConfigTails](posix_and_gnu_args_tails, skip: 1)!
+	assert config6.mix == false
+	assert config6.paths == ['tail0', 'tail1', 'tail2']
+	assert no_matches6 == []
+
+	config7, no_matches7 := flag.to_struct[ConfigTail](posix_and_gnu_args_no_tail)!
+	assert config7.mix == false
+	assert config7.path == ''
+	assert no_matches7 == ['-f']
+
+	config8, no_matches8 := flag.to_struct[ConfigTail](posix_and_gnu_args_no_tail, skip: 1)!
+	assert config8.mix == false
+	assert config8.path == ''
+	assert no_matches8 == []
+}