From 31dce016f4c0eadb19ac0c513aff0d3c2fdd901f Mon Sep 17 00:00:00 2001
From: Matthew Fernandez <matthew.fernandez@gmail.com>
Date: Mon, 20 Jun 2022 19:49:12 -0700
Subject: [PATCH] add a test case for #121

---
 tests/121.dot            | 26 ++++++++++++++++++++++++++
 tests/test_regression.py | 14 ++++++++++++++
 2 files changed, 40 insertions(+)
 create mode 100644 tests/121.dot

diff --git a/tests/121.dot b/tests/121.dot
new file mode 100644
index 000000000..e6efade4c
--- /dev/null
+++ b/tests/121.dot
@@ -0,0 +1,26 @@
+digraph G
+{
+  subgraph cluster_G1
+  {
+    {
+      rank=same;
+      A -> B;
+    }
+
+    C -> A;
+  }
+
+  subgraph cluster_G2
+  {
+    {
+      rank=same;
+      D -> E;
+    }
+
+    F -> E;
+  }
+
+  B -> D [ constraint=none ];
+  B -> D [ constraint=none ];
+}
+
diff --git a/tests/test_regression.py b/tests/test_regression.py
index 5a6c4aa78..35cdb313c 100644
--- a/tests/test_regression.py
+++ b/tests/test_regression.py
@@ -104,6 +104,20 @@ def test_56():
   # process it with Graphviz
   dot("svg", input)
 
+@pytest.mark.xfail()
+def test_121():
+  """
+  test a graph that previously caused an assertion failure in `merge_chain`
+  https://gitlab.com/graphviz/graphviz/-/issues/121
+  """
+
+  # locate our associated test case in this directory
+  input = Path(__file__).parent / "121.dot"
+  assert input.exists(), "unexpectedly missing test case"
+
+  # process it with Graphviz
+  dot("pdf", input)
+
 def test_131():
   """
   PIC back end should produce valid output
-- 
2.40.0