`transitiveclosure` only works on `SimpleDiGraph`s

[gdalle]

The guilty lines of code:

https://github.com/JuliaGraphs/Graphs.jl/blob/53cb58152a19ca62fa01439300948c30035dca6a/src/digraph/transitivity.jl#L105-L108

Is there a reason why:

• the function dispatches on a concrete graph type
• it doesn't support undirected graphs?

First spotted by https://github.com/JuliaGraphs/SimpleWeightedGraphs.jl/issues/10

[jlapeyre]

Maybe the only method in the package for transitiveclosure! requires SimpleDiGraph so the author did not want to provide a method that would throw an error when calling the inner function. But Graphs is intended to be extended outside the package. So if someone implements a graph type and a method for transitiveclosure! then they would not get transitiveclosure for free.

[simonschoelly]

That function is kind of similar to other graph operators that we have, such as blockdiag in that they relay on add_edge! to add edges - which is difficult if we need to have some kind of metadata to construct an edge.

That was probably the reason that it was restricted to SimpleDiGraph - or this function was written at a time when there was no abstract graph interface.

Using transitiveclosure on an undirected graph would just convert all connected components into complete graphs - so the usefulness of that case is limited.

[gdalle]

Yes but it could be a directed AbstractGraph at least

[gdalle]

Linked to #281