Fix for sync error when the ireqs being merged have no names

When no name was provided, the ireqs would have no underlying req
attribute. Calling ireq.specifier would raise an AttributeError.

piptools/sync.py

@@ -104,7 +104,11 @@ def merge(
                 if existing_ireq:
                     # NOTE: We check equality here since we can assume that the
                     # requirements are all pinned
-                    if ireq.specifier != existing_ireq.specifier:
+                    if (
+                        ireq.req
+                        and existing_ireq.req
+                        and ireq.specifier != existing_ireq.specifier
+                    ):
                         raise IncompatibleRequirements(ireq, existing_ireq)
 
             # TODO: Always pick the largest specifier in case of a conflict

tests/test_sync.py

@@ -115,6 +115,26 @@ def test_merge_urls(from_line):
     )
 
 
+@pytest.mark.parametrize(
+    "install_req",
+    (
+        "from_line",
+        "from_editable",
+    ),
+)
+def test_merge_no_name_urls(install_req, request):
+    install_req = request.getfixturevalue(install_req)
+    url = "file:///example.zip"
+    requirements = [
+        install_req(url),
+        install_req(url),
+    ]
+
+    assert Counter(requirements[1:]) == Counter(
+        merge(requirements, ignore_conflicts=False)
+    )
+
+
 def test_diff_should_do_nothing():
     installed = []  # empty env
     reqs = []  # no requirements