[BUG] `topological_sort` returns duplicate vertices

[stev47]

Description of bug
topological_sort may return duplicate vertices

How to reproduce

g = SimpleDiGraph([Edge(2 => 1)])
topological_sort(g)

Expected behavior

[2, 1]

Actual behavior

[2, 1, 1]

Code demonstrating bug
see above

Version information

julia> versioninfo()
Julia Version 1.8.3
Commit 0434deb161e (2022-11-14 20:14 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD A10-7860K Radeon R7, 12 Compute Cores 4C+8G
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, bdver3)
  Threads: 1 on 4 virtual cores
Environment:
  JULIA_PKG_USE_CLI_GIT = 1

(@v1.8) pkg> status Graphs
Status `~/.julia/environments/v1.8/Project.toml`
  [86223c79] Graphs v1.7.4 `~/.julia/dev/Graphs`

Additional context
none

[simonschoelly]

Uh, this is bad. topological_sort is defined in the Experimental submodule (where correctness is less guaranteed) and should not be exported outside, but it is, because we import the topological_sort symbol from outside.

In the meantime, one should probably use topologial_sort_by_dfs - that one seems to work correctly, at least for that particular case.

[simonschoelly]

A possible fix can be found here: #214

[gdalle]

This is fixed

julia> g = SimpleDiGraph([Edge(2 => 1)])
{2, 1} directed simple Int64 graph

julia> topological_sort(g)
2-element Vector{Int64}:
 2
 1