fix

vlib/v/checker/fn.v

@@ -3366,7 +3366,7 @@ fn (mut c Checker) array_builtin_method_call(mut node ast.CallExpr, left_type as
 
 fn (mut c Checker) check_for_mut_receiver(mut expr ast.Expr) (string, token.Pos) {
 	to_lock, pos := c.fail_if_immutable(mut expr)
-	if !expr.is_lvalue() {
+	if !expr.is_lvalue() && expr !is ast.CallExpr {
 		c.error('cannot pass expression as `mut`', expr.pos())
 	}
 	return to_lock, pos

vlib/v/tests/fns/method_call_chain_mut_test.v

@@ -0,0 +1,54 @@
+struct App {
+	name    string
+	version string
+mut:
+	commands []Command
+	flags    []Flag
+}
+
+struct Command {
+	name    string
+	execute fn (cmd Command) = unsafe { nil }
+}
+
+struct Flag {
+	name     string
+	abbrev   string
+	required bool
+}
+
+fn (mut app App) command(cmd Command) &App {
+	app.commands << cmd
+	return unsafe { app }
+}
+
+fn (mut app App) flag(flag Flag) &App {
+	app.flags << flag
+	return unsafe { app }
+}
+
+fn test_main() {
+	mut app := App{
+		name:     'amazing cli'
+		version:  '0.1.0'
+		commands: []
+		flags:    []
+	}
+
+	w := app.command(Command{
+		name:    'test'
+		execute: fn (cmd Command) {
+			println('Testing')
+		}
+	}).flag(Flag{
+		name:     'verbose'
+		abbrev:   'v'
+		required: true
+	})
+
+	println(app)
+	assert app.flags.len == 1
+	assert app.commands.len == 1
+	assert w.flags.len == 1
+	assert w.commands.len == 1
+}