Clear base fragments when clicking empty references

src/hyperlink/_url.py

@@ -1617,7 +1617,7 @@ def click(self, href=u""):
             if clicked.absolute:
                 return clicked
         else:
-            clicked = self
+            clicked = self.replace(fragment=u"")
 
         query = clicked.query
         if clicked.scheme and not clicked.rooted:

src/hyperlink/test/test_decoded_url.py

@@ -15,6 +15,17 @@
 
 
 class TestURL(HyperlinkTestCase):
+    def test_click_empty_reference_clears_fragment(self):
+        # type: () -> None
+        base = DecodedURL.from_text("http://example.com/a/b?x=1#old")
+        expected = "http://example.com/a/b?x=1"
+        assert base.click("").to_text() == expected
+        assert base.click().to_text() == expected
+        assert base.click("") == base.click(URL())
+        assert base.click("") == base.click(DecodedURL())
+        assert base.click("#new").fragment == "new"
+        assert base.fragment == "old"
+
     def test_durl_basic(self):
         # type: () -> None
         bdurl = DecodedURL.from_text(BASIC_URL)

src/hyperlink/test/test_url.py

@@ -474,6 +474,21 @@ def test_click(self):
         u3 = u.click(u2)
         self.assertEqual(u3.to_text(), "http://localhost/foo/bar")
 
+    def test_click_empty_reference_clears_fragment(self):
+        # type: () -> None
+        for text, expected in (
+            ("http://example.com/a/b?x=1#old", "http://example.com/a/b?x=1"),
+            ("http://example.com/a/../b#old", "http://example.com/b"),
+            ("http://example.com/#old", "http://example.com/"),
+        ):
+            base = URL.from_text(text)
+            assert base.click("").to_text() == expected
+            assert base.click().to_text() == expected
+            assert base.click("") == base.click(URL())
+            assert base.click("#new").fragment == "new"
+            assert base.click("#").fragment == ""
+            assert base.fragment == "old"
+
     def test_clickRFC3986(self):
         # type: () -> None
         """