And for anyone (like me) that thought, "ok that's fair enough, what's the problem?", the issue is that it could also be interpreted as generic arguments.

A full explanation of the ambiguity is in the linked issue:

  let a = (b<c, d>(e));

  This can either mean a tuple

  let a = ((b < c), (d > e));

  Or a pair of generic arguments.

  let a = b::<c, d>(e);