src/Container.php
protected function turnIntoResolvableClosure($abstract, $concrete)
    {
        return function (Container $container, $parameters = []) use ($abstract, $concrete) {
            return ($abstract == $concrete ? $container->resolve($abstract)
                : $container->resolve($concrete, $parameters));
        };
    }
protected function turnIntoResolvableClosure($abstract, $concrete)
    {
        return function (Container $container, $parameters = []) use ($abstract, $concrete) {
            return ($abstract == $concrete ? $container->resolve($abstract)
                : $container->resolve($concrete, $parameters));
        };
    }
Turn class name into resolvable closure.

@param string $abstract The class name
@param \Closure|string $concrete Can be instance of \Closure or class name.
@return \Closure