Allow editable installs to satisfy direct-URL dependencies

news/10216.bugfix.rst

@@ -0,0 +1,2 @@
+Allow installing a package as editable alongside another package that
+depends on it, via a direct URL reference to the same location.

src/pip/_internal/resolution/resolvelib/factory.py

@@ -219,6 +219,8 @@ def _make_base_candidate_from_link(
                     self._build_failures[link] = e
                     return None
 
+            return self._editable_candidate_cache[link]
+        elif link in self._editable_candidate_cache:
             return self._editable_candidate_cache[link]
         else:
             if link not in self._link_candidate_cache:

tests/functional/test_new_resolver.py

@@ -328,6 +328,53 @@ def test_new_resolver_installs_editable(script: PipTestEnvironment) -> None:
     script.assert_installed_editable("dep")
 
 
+def test_new_resolver_editable_satisfies_direct_url_dep(
+    script: PipTestEnvironment,
+) -> None:
+    """Regression test for https://github.com/pypa/pip/issues/10216.
+
+    Installing ``-e A -e B`` used to fail with ``ResolutionImpossible`` when ``A``
+    depends on ``B`` via a direct URL (PEP 440) reference pointing at the same
+    location as the editable ``B``:
+
+    > Cannot install package-a and package-b because these package versions have
+    > conflicting dependencies.
+
+    This was because the resolver created both an ``EditableCandidate`` (from the
+    user-supplied ``-e B``) and a ``LinkCandidate`` (from ``A``'s transitive
+    direct-URL dependency) for the same link, and treated them as conflicting.
+    """
+    dep_path = create_test_package_with_setup(script, name="dep", version="0.1.0")
+    dep_url = dep_path.as_uri()
+    base_path = script.scratch_path / "base"
+    base_path.mkdir()
+    base_path.joinpath("setup.py").write_text(
+        textwrap.dedent(
+            f"""
+            from setuptools import setup
+            setup(
+                name="base",
+                version="0.1.0",
+                install_requires=["dep @ {dep_url}"],
+            )
+            """
+        )
+    )
+    script.pip(
+        "install",
+        "--no-build-isolation",
+        "--no-cache-dir",
+        "--no-index",
+        "-e",
+        base_path,
+        "-e",
+        dep_path,
+    )
+    script.assert_installed(base="0.1.0", dep="0.1.0")
+    script.assert_installed_editable("base")
+    script.assert_installed_editable("dep")
+
+
 @pytest.mark.parametrize(
     "requires_python, ignore_requires_python, dep_version",
     [