The infix syntax is not great, but you can at least write frac(n (n + 1), 2) and it renders the way you would expect.

Edit:

This is not even an option in typst, which uses the space as an escape character.

Technically LaTeX does this too with omitted curly braces:

\sum_{n=1}^9   % works
\sum_{n=1}^10  % doesn't work

Everyone just quickly learns to not do that.