With a bit of fiddeling you can use every kind of bracket to basically make function calls in c++
Here are the links to the snippets, all compile and run
Rust: https://godbolt.org/z/bM3d7bG14
Yeah, I always wonder, if it was an intentional decision or just some first draft that stayed in.
Because I do think, it's kind of smart. Some macros make sense to call with either () or {}, depending on if you pass a code snippet that's multi-line or not.
So, you could come up with a complicated syntax to define which bracket types are acceptable, or you could just not bother. 🙃