Disallow merging 0-length spans

spacy/errors.py

@@ -567,6 +567,7 @@ class Errors(object):
     E197 = ("Row out of bounds, unable to add row {row} for key {key}.")
     E198 = ("Unable to return {n} most similar vectors for the current vectors "
             "table, which contains {n_rows} vectors.")
+    E199 = ("Unable to merge 0-length span at doc[{start}:{end}].")
 
 
 @add_codes

spacy/tests/doc/test_retokenize_merge.py

@@ -425,3 +425,10 @@ def test_retokenize_skip_duplicates(en_vocab):
         retokenizer.merge(doc[0:2])
     assert len(doc) == 2
     assert doc[0].text == "hello world"
+
+
+def test_retokenize_disallow_zero_length(en_vocab):
+    doc = Doc(en_vocab, words=["hello", "world", "!"])
+    with pytest.raises(ValueError):
+        with doc.retokenize() as retokenizer:
+            retokenizer.merge(doc[1:1])

spacy/tokens/_retokenize.pyx

@@ -55,6 +55,8 @@ cdef class Retokenizer:
         """
         if (span.start, span.end) in self._spans_to_merge:
             return
+        if span.end - span.start <= 0:
+            raise ValueError(Errors.E199.format(start=span.start, end=span.end))
         for token in span:
             if token.i in self.tokens_to_merge:
                 raise ValueError(Errors.E102.format(token=repr(token)))