Proposal: empty-var args should dispatch to most general method

[oxinabox]

This is a proposal for consideration for whenever we get around to Julia 2.0.
Writing it here so it is not lost to time.
This would resolve #24315 (though that issue is labelled [docs] so I am making a new one for a non-docs based solution.)

In julia the normal rule of dispatch is to the most specific method.
This even applies when that method becomes the most specific due to having varargs, and when those varargs are not passed:

julia> bar(xs::Int...) = 1
bar (generic function with 1 method)

julia> bar(xs...) = 2
bar (generic function with 2 methods)

julia> bar()
1

Now there are a few options for how to resolve this (when we are considering that breaking is an option).

1. As we currently do: empty-varargs go to most specific method, even if we have not passed in any of them.
2. Change Vararg so it only matches 1+ arguments.
3. Make this an ambiguity error
4. Dispatch to most general vararg constraint (this proposal)

I would like to make the argument that empty varargs should always go to the most general way of meeting that constraint.
Inverting the normal rules.
The reason for this is not so complex:
The whole motivation of adding a dispatch with a specific type, is that you can use insights based on knowing your object has that type to write a better method.
For some definition of better: whether faster, more correct, etc.
But if you are given no actual instance of your type, then there is no way you have an additional insight -- if you did that insight would be applicable in every case no matter the argument type, so i would expect it to be in the more general method that already existed before you brought up your type.
And this change would allow that method to be hit.

The downside is this is another rule to remember, and implement.
The upside is it would remove a lot of cases of accidental type-piracy.
And doesn't introduce more ambiguity errors, nor require writing extra methods.

[stevengj]

See #42455 for a more general, less-breaking variant of this. For the particular case above, you could define bar(xs::VarargOne{Int,Int,N}) where {N} = 1 in my proposal to require at least 1 Int argument.

[vtjnash]

Duplicate of #795