Like huon said, this is already the case, so I don't see how it could be confusing. The compiler would infer all of that and complain if the constraints are not

Like huon said, this is already the case, so I don't see how it could be confusing. The compiler would infer all of that and complain if the constraints are not satisfied. A literal fn syntax would be pretty cool, but I have doubts it'd ever be accepted, which is why I figured closure syntax could be adapted to allow this use case.