Progress on #2080

[toots]

This PR makes some progress on #2080.

It is based on the observation that this works:

def f(x) = [x, {foo = "aabb"}] end
f(())

but this does not:

def f(x) = [{foo = "aabb"},x] end
f(())

For some reason, typechecking the variables for a list first allows for the sup mechanism to be applied when calling the function so this PR does just that.

Perhaps this could shed more light on how to properly fix the issue?