Related problem

We have the concept about closure and block, but currently closure syntax can be the same to block, which leads to confusing behavior.

Like: { a a }, should it be a closure or block?

Describe the solution you'd like

I'd like to restrict closure syntax to be something like {|| 3 * 3}, or {|arg| arg * arg}

Describe alternatives you've considered

No response

Additional context and details

Actually I think it's relative to issue: #7575

I have a easy solution for it(check #8274), but it required closure to have more unique syntax.. or else the ut is broken