fix: honor TakesFile for every flag type in fish completion

fish.go

@@ -161,15 +161,9 @@ func prepareFishFlags(binary string, owner *Command) []string {
 }
 
 func fishAddFileFlag(flag Flag, completion *strings.Builder) {
-	switch f := flag.(type) {
-	case *StringFlag:
-		if f.TakesFile {
-			return
-		}
-	case *StringSliceFlag:
-		if f.TakesFile {
-			return
-		}
+	// TakesFile is a field of every flag type, not only StringFlag and StringSliceFlag
+	if f, ok := flag.(interface{ takesFile() bool }); ok && f.takesFile() {
+		return
 	}
 	completion.WriteString(" -f")
 }

fish_test.go

@@ -107,6 +107,29 @@ func TestFishCompletionShellComplete(t *testing.T) {
 	assert.Contains(t, res, fmt.Sprintf("complete -c greet -n '__fish_seen_subcommand_from config c; and __fish_seen_subcommand_from sub-config s ss' -xa '(greet config sub-config %s 2>/dev/null)'", completionFlag))
 }
 
+func TestFishCompletionTakesFileForAllFlagTypes(t *testing.T) {
+	cmd := &Command{
+		Name: "greet",
+		Flags: []Flag{
+			&StringFlag{Name: "config", TakesFile: true},
+			&GenericFlag{Name: "input", Value: &Parser{}, TakesFile: true},
+			&StringMapFlag{Name: "env-file", TakesFile: true},
+			&StringFlag{Name: "name"},
+			// a boolean takes no value, so there is no file to complete
+			&BoolFlag{Name: "dry-run", TakesFile: true},
+		},
+	}
+
+	res, err := cmd.ToFishCompletion()
+	require.NoError(t, err)
+
+	assert.Contains(t, res, "'__fish_greet_no_subcommand' -l config -r")
+	assert.Contains(t, res, "'__fish_greet_no_subcommand' -l input -r")
+	assert.Contains(t, res, "'__fish_greet_no_subcommand' -l env-file -r")
+	assert.Contains(t, res, "'__fish_greet_no_subcommand' -f -l name -r")
+	assert.Contains(t, res, "'__fish_greet_no_subcommand' -f -l dry-run")
+}
+
 func TestFishCompletionUsesFirstLineOfUsage(t *testing.T) {
 	cmd := &Command{
 		Name: "greet",

flag_impl.go

@@ -366,6 +366,11 @@ func (f *FlagBase[T, C, V]) TakesValue() bool {
 	return reflect.TypeOf(t) == nil || reflect.TypeOf(t).Kind() != reflect.Bool
 }
 
+// takesFile returns true if the flag's value is a file path, for shell completion
+func (f *FlagBase[T, C, V]) takesFile() bool {
+	return f.TakesFile && f.TakesValue()
+}
+
 // GetDefaultText returns the default text for this flag
 func (f *FlagBase[T, C, V]) GetDefaultText() string {
 	return f.DefaultText