WIP: remove fallback eltype that returns Any for all types

[JeffBezanson]

For a long time we've allowed eltype to be called on anything and return Any (eltype(:x), eltype(+), you name it). This feels very julia 0.1 to me and doesn't seem like a good thing.

Removing this method already uncovered a couple bugs, where iterators were silently and incorrectly getting Any from eltype because the wrong method was called.

The part of this I'm not sure about is the Random code, where the Sampler types have an eltype parameter, and call eltype on a constructor argument to populate it. Some of the types passed here do not have eltype methods, and indeed the tests pass if I just replace the eltype calls with Any. So maybe this type parameter is unused and can just be removed. cc @rfourquet

Currently I have the fallback eltype give a MethodError, but it could be a deprecation warning instead.

[vtjnash]

?

aren't these supposed to be equivalent, with the subtyping test likely being more reliable

[JeffBezanson]

Type{<:Array{T}} where T is a strict subtype of Type{<:Array}, since no value of T turns Array{T} into Array.

[vtjnash]

need to now handle the cases where !@isdefined K or !@isdefined V

[JeffBezanson]

Yes, and now I notice this can actually be deleted since it's covered by the eltype method for AbstractDict.

[StefanKarpinski]

Triage accepts.

[mauro3]

Why not give a deprecation instead of a method error?

[JeffBezanson]

So, the real issue here seems to be that the default value of IteratorEltype is HasEltype. Without a fallback eltype, it would make more sense to make that EltypeUnknown, but then every type that defines eltype has to add a definition IteratorEltype(::Type{MyType}) = HasEltype(), and things will be pretty broken until they do. So this change might not actually be worth it.

Of course, IteratorSize is similar in that the default is HasLength, but that seems to be much less of an issue in practice than eltype.

[StefanKarpinski]

Wouldn't it make sense to change the default to not having an eltype? That seems like a strange default to me.

[JeffBezanson]

It's a nuisance; every time you define eltype you'd also have to define IteratorEltype for it to "take effect". But with HasEltype as the default, you get a method error for eltype, and at that point you can decide whether to define eltype or IteratorEltype(::Type{MyType}) = EltypeUnknown(). So it works surprisingly well. I'm not sure we should touch this until we have a better solution e.g. based on applicable.