Should iteration over Associatives return Pairs

[Keno]

Right now we have

julia> first(Dict(1=>2))
(1,2)

I'm proposing this be changed to

julia> first(Dict(1=>2))
1 => 2

i.e. treating dicts as a list of key value pairs for the purposes of iteration. Making them a pair makes clear what the return value means. I think this is a fairly non-breaking change as Pair essentially behaves as a 2-element tuple anyway.

[StefanKarpinski]

+10, I feel like this was just a possible oversight in the Pair change, or maybe there was some compatibility reason for it.

[stevengj]

Seems reasonable to me; for (k,v) in dict and similar should still work, as @Keno says.

[lindahua]

+1

[ivarne]

+1

Ref #9905 for previous discussion and oversight.

[JeffBezanson]

This is fine but there's an interesting point here regarding covariance. If the eltype of a Dict{Any,Any} is Tuple{Any,Any}, it can return more specific tuples like Tuple{Int,Int} for particular elements. With Pairs, every item iterated will have to be a Pair{Any,Any} exactly.

[JeffBezanson]

To be a bit more concrete, returning k=>v as an element of a Dict might be wrong. One needs to be careful to return a Pair{K,V}(k,v).

[StefanKarpinski]

I'm not sure what the practical consequences of that would be...

[JeffBezanson]

Not many, but for example 1=>2 is === to an element of Dict(1=>2), but not to an element of Dict{Any,Any}(1=>2).

[ivarne]

Ref #12319.

We should consider making this change universal for all functions that return key => value pairs.