I don't see how (: something) is different from (something) in terms of ambiguity, both can be ambiguous to the compiler for the same reasons.
I'm also not against such an addition at all. All I was saying is, we already solved it in a nice and clean way and we should go that path again, but we solved it for a different not purely sugar purpose. However this pitch is pure sugar and discussions from the past showed that the core team won't accept such proposals even for a review at this age of Swift.
Here is what @Chris_Lattner3 said back then.