diff --git a/src/pip/_internal/resolution/resolvelib/candidates.py b/src/pip/_internal/resolution/resolvelib/candidates.py
index 6abadf04d..3f431843f 100644
--- a/src/pip/_internal/resolution/resolvelib/candidates.py
+++ b/src/pip/_internal/resolution/resolvelib/candidates.py
@@ -28,6 +28,12 @@ if MYPY_CHECK_RUNNING:
     from .base import Requirement
     from .factory import Factory
 
+    BaseCandidate = Union[
+        "AlreadyInstalledCandidate",
+        "EditableCandidate",
+        "LinkCandidate",
+    ]
+
 
 logger = logging.getLogger(__name__)
 
@@ -54,7 +60,10 @@ def make_install_req_from_editable(link, parent):
     assert parent.editable, "parent not editable"
     return install_req_from_editable(
         link.url,
-        comes_from=parent.comes_from,
+        # HACK: install_req_from_editable accepts Optional[str] here, but
+        # parent.comes_from is Union[str, InstallRequirement, None]. How do
+        # we fix the type hint conflicts?
+        comes_from=parent.comes_from,  # type: ignore
         use_pep517=parent.use_pep517,
         isolated=parent.isolated,
         constraint=parent.constraint,
@@ -302,7 +311,7 @@ class ExtrasCandidate(Candidate):
     """
     def __init__(
         self,
-        base,  # type: Union[AlreadyInstalledCandidate, LinkCandidate]
+        base,  # type: BaseCandidate
         extras,  # type: Set[str]
     ):
         # type: (...) -> None
diff --git a/src/pip/_internal/resolution/resolvelib/factory.py b/src/pip/_internal/resolution/resolvelib/factory.py
index 705f47f13..cf5b2b19e 100644
--- a/src/pip/_internal/resolution/resolvelib/factory.py
+++ b/src/pip/_internal/resolution/resolvelib/factory.py
@@ -34,6 +34,7 @@ if MYPY_CHECK_RUNNING:
     from pip._internal.resolution.base import InstallRequirementProvider
 
     from .base import Candidate, Requirement
+    from .candidates import BaseCandidate
 
     C = TypeVar("C")
     Cache = Dict[Link, C]
@@ -85,7 +86,7 @@ class Factory(object):
                 self._editable_candidate_cache[link] = EditableCandidate(
                     link, parent, factory=self, name=name, version=version,
                 )
-            base = self._editable_candidate_cache[link]
+            base = self._editable_candidate_cache[link]  # type: BaseCandidate
         else:
             if link not in self._link_candidate_cache:
                 self._link_candidate_cache[link] = LinkCandidate(