Unify reshape routing to make extension easier

[darsnack]

We ran into an issue trying to extend Base.reshape in FluxML/Flux.jl#1448. Ideally, someone should only have to overload

Base.reshape(x::MySpecialArray, dims::Dims)

to get insert their custom behavior. Currently, calls involving a single : bypass this method signature directly to the internal _reshape. I'd like to redefine this line to be:

reshape(parent::AbstractArray, dims::Tuple{Vararg{Union{Int,Colon}}}) = reshape(parent, _reshape_uncolon(parent, dims))

so that it routes through the signature above after the uncolon logic is applied. I see no real problems with this, but I wanted to create an issue before a PR to get told why I'm wrong.

[mbauman]

This is actually something I've wanted to do for a long time. It's a bit of a rats nest right now, and IIRC there are two levels of redirection — one for colons and one for axes/offset arrays. It's the latter that poses the bigger problem.

[darsnack]

I assume the second level is this? So would you want to lift that logic out of an internal function?

[mbauman]

Yes, I'd think so. But it's a general problem with Offset arrays — general array types want the "extension" method to be the simple ::Dims, whereas offset arrays require ::Axes. It makes things go in circles.

[darsnack]

Okay I see why there was this extra path now. There's a dispatch ambiguity if you redefine it like I want mentioned above. Maybe we can just tweak _reshape_uncolon to no-op on the no colon case? Then we just remove reshape(parent, ::Dims).

[darsnack]

It looks like there was already a fix for the initial complaint. I can still keep this issue open since we still want to clean up the interface.