Fix empty-title wikilinks being incorrectly parsed as Wikilink nodes

MediaWiki does not treat [[]] or [[|...]] as wikilinks because their
title is empty. Calls to filter_wikilinks() on text containing these
sequences returned false positives.

The fix is in builder.py (pure Python, runs for both the C and Python
tokenizers): when a WikilinkClose token is encountered and the resulting
title is an empty string, the node is demoted to a plain Text node
containing the original bracket sequence instead of being wrapped in a
Wikilink.

Fixes #292.

src/mwparserfromhell/parser/builder.py

@@ -162,8 +162,14 @@ class Builder:
                 self._push()
             elif isinstance(token, tokens.WikilinkClose):
                 if title is not None:
-                    return Wikilink(title, self._pop())
-                return Wikilink(self._pop())
+                    wikilink = Wikilink(title, self._pop())
+                else:
+                    wikilink = Wikilink(self._pop())
+                # MediaWiki treats [[]] and [[|...]] (empty title) as plain
+                # text rather than actual wikilinks.
+                if not str(wikilink.title):
+                    return Text(str(wikilink))
+                return wikilink
             else:
                 self._write(self._handle_token(token))
         raise ParserError("_handle_wikilink() missed a close token")

tests/test_parser.py

@@ -102,3 +102,24 @@ def test_skip_style_tags(pyparser):
     without_style = parser.Parser().parse(text, skip_style_tags=True)
     assert_wikicode_equal(a, with_style)
     assert_wikicode_equal(b, without_style)
+
+
+@pytest.mark.parametrize(
+    "text",
+    [
+        "[[]]",
+        "[[|]]",
+        "[[|foo]]",
+        "[[|||]]",
+    ],
+)
+def test_empty_title_wikilink_is_text(text):
+    """[[]] and [[|...]] have an empty title and should not be wikilinks.
+
+    MediaWiki does not render these as hyperlinks; the parser must treat them
+    as plain text so that filter_wikilinks() does not return false positives.
+    Regression test for https://github.com/earwig/mwparserfromhell/issues/292.
+    """
+    parsed = parser.Parser().parse(text)
+    assert parsed.filter_wikilinks() == []
+    assert str(parsed) == text