return function(ContainerConfigurator $configurator) {
    $services = $configurator->services()
        ->defaults()
        ->bind(Foo::class.' $foo', inline_service(Foo::class))
    ;
};

Currently it throws Invalid binding for service "": named arguments must start with a "$", and FQCN must map to references. Neither applies to binding "Foo $foo", while binding inline service just by name works.