Why viewers are angry about Matt Damon's new action blockbuster

The trailer for Matt Damon's next film, the action-adventure blockbuster The Great Wall, debuted over the weekend - and it's already been met with an intense backlash.

In the film, Damon plays a warrior defending The Great Wall of China from mysterious monsters.

Quite what an American is doing fighting among legions of Chinese warriors in medieval China isn't explained in the trailer - but it's already seen the film accused of white washing and racism for not casting a Chinese lead.

Taiwanese-American actress Constance Wu, best known for her role in sitcom Fresh Off The Boat, criticised the film in a lengthy Twitter post.

She called on Hollywood to "stop perpetuating the racist myth that [only a] white man can save the world. It's not based in actual fact."

"Our heroes don't look like Matt Damon. They look like Malala. Ghandi (sic). Mandela. Your big sister when she stood up for you to those bullies that one time. We don't need salvation. We like our colour and our culture and our strengths and our own stories," she wrote.

While Asian actors round out the supporting cast of the joint Chinese-American blockbuster, many of the lead roles are played by American actors, including Damon, Willem Dafoe and Chilean-born Game of Thrones actor Pedro Pascal.

It's the latest in a long line of Hollywood films casting white actors in what would traditionally be Asian roles.

Tilda Swinton has received criticism for starring as the Tibetan monk 'Ancient One' in Marvel's Doctor Strange.

Scarlett Johansson plays Major Motoko Kusanagi in the upcoming big screen manga adaptation The Ghost In The Shell, while Tilda Swinton will play the role of Tibetan monk 'Ancient One' in Marvel's Doctor Strange, set for release in November.

The issue isn't confined to Asian roles: In 2015, writer/director Cameron Crowe infamously managed to make a romantic comedy set on Hawaii, titled Aloha, without casting a single Hawaiian person in a lead role - and even casting Emma Stone as a "half-Chinese" character.

It remains to be seen how The Great Wall will explain Damon's presence in ancient China, but as the top-rated comment under the film's YouTube trailer points out: "This is gonna be weird if Matt Damon has Chinese parents in the movie..."