Add xfail test for deprojectivization SBD bug

spacy/tests/regression/test_issue2772.py

@@ -0,0 +1,19 @@
+'''Test that deprojectivization doesn't mess up sentence boundaries.'''
+import pytest
+from ...syntax.nonproj import projectivize, deprojectivize
+from ..util import get_doc
+
+@pytest.mark.xfail
+def test_issue2772(en_vocab):
+    words = 'When we write or communicate virtually , we can hide our true feelings .'.split()
+    # A tree with a non-projective (i.e. crossing) arc
+    # The arcs (0, 4) and (2, 9) cross.
+    heads = [4, 1, 7, -1, -1, -1, 3, 2, 1, 0, 2, 1, -1, -1]
+    deps = ['dep'] * len(heads)
+    heads, deps = projectivize(heads, deps)
+    doc = get_doc(en_vocab, words=words, heads=heads, deps=deps)
+    assert doc[0].is_sent_start == True
+    assert doc[1].is_sent_start is None
+    deprojectivize(doc)
+    assert doc[0].is_sent_start == True
+    assert doc[1].is_sent_start is None