degree(::Graph{Directed}, ::Int) does not count in-neighbors

[YueRen]

There is a bug in degree(::Graph, ::Int) function, which you can check by running

using Oscar
G = Graph{Directed}(3)
add_edge!(G,1,2)
add_edge!(G,2,3)
degree(G,2) # returns 1, not 2

I will prepare a pull-request shortly.

[benlorenz]

see #4441 for details, we can keep this ticket open after #4441 is merged and discuss this further during triage.

[fingolfin]

We discussed, degree needs to stay in its current behavior (otherwise it'd be breaking) but we should add functions to get the in and out degree (in_degree and out_degree? For tab completion, degree_in might be nicer`???). And then the docstrings should point to each others.

[lgoettgens]

We could also deprecate degree(G::Graph{Directed}) in favor of outdegree(G) to somehow notify users of this weird current behaviour

[JohnAAbbott]

Not sure if it is a good idea, but... we could have both in_degree and degree_in so that TAB completion is helpful and so that we have a mnemonic name. In the discussion it was stated that currently directed graphs in OSCAR must be "simple" (i.e. no loops, and no multiple edges); it would be good if the design of the functions is flexible enough to permit possible future extensions to the sorts of graph OSCAR can handle.