diff --git a/src/pip/_internal/resolution/resolvelib/factory.py b/src/pip/_internal/resolution/resolvelib/factory.py
index 145bdbf71a1..09437cffa30 100644
--- a/src/pip/_internal/resolution/resolvelib/factory.py
+++ b/src/pip/_internal/resolution/resolvelib/factory.py
@@ -742,7 +742,7 @@ def get_installation_error(
         # The simplest case is when we have *one* cause that can't be
         # satisfied. We just report that case.
         if len(e.causes) == 1:
-            req, parent = e.causes[0]
+            req, parent = next(iter(e.causes))
             if req.name not in constraints:
                 return self._report_single_requirement_conflict(req, parent)
 
diff --git a/src/pip/_internal/resolution/resolvelib/reporter.py b/src/pip/_internal/resolution/resolvelib/reporter.py
index 0594569d850..587b15d1ca1 100644
--- a/src/pip/_internal/resolution/resolvelib/reporter.py
+++ b/src/pip/_internal/resolution/resolvelib/reporter.py
@@ -1,6 +1,6 @@
 from collections import defaultdict
 from logging import getLogger
-from typing import Any, DefaultDict
+from typing import Any, DefaultDict, Optional
 
 from pip._vendor.resolvelib.reporters import BaseReporter
 
@@ -9,7 +9,7 @@
 logger = getLogger(__name__)
 
 
-class PipReporter(BaseReporter):
+class PipReporter(BaseReporter[Requirement, Candidate, Any]):
     def __init__(self) -> None:
         self.reject_count_by_package: DefaultDict[str, int] = defaultdict(int)
 
@@ -55,7 +55,7 @@ def rejecting_candidate(self, criterion: Any, candidate: Candidate) -> None:
         logger.debug(msg)
 
 
-class PipDebuggingReporter(BaseReporter):
+class PipDebuggingReporter(BaseReporter[Requirement, Candidate, Any]):
     """A reporter that does an info log for every event it sees."""
 
     def starting(self) -> None:
@@ -71,7 +71,9 @@ def ending_round(self, index: int, state: Any) -> None:
     def ending(self, state: Any) -> None:
         logger.info("Reporter.ending(%r)", state)
 
-    def adding_requirement(self, requirement: Requirement, parent: Candidate) -> None:
+    def adding_requirement(
+        self, requirement: Requirement, parent: Optional[Candidate]
+    ) -> None:
         logger.info("Reporter.adding_requirement(%r, %r)", requirement, parent)
 
     def rejecting_candidate(self, criterion: Any, candidate: Candidate) -> None:
diff --git a/src/pip/_internal/resolution/resolvelib/resolver.py b/src/pip/_internal/resolution/resolvelib/resolver.py
index c12beef0b2a..07dcb766b2c 100644
--- a/src/pip/_internal/resolution/resolvelib/resolver.py
+++ b/src/pip/_internal/resolution/resolvelib/resolver.py
@@ -2,7 +2,7 @@
 import functools
 import logging
 import os
-from typing import TYPE_CHECKING, Dict, List, Optional, Set, Tuple, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Set, Tuple, cast
 
 from pip._vendor.packaging.utils import canonicalize_name
 from pip._vendor.resolvelib import BaseReporter, ResolutionImpossible
@@ -82,7 +82,7 @@ def resolve(
             user_requested=collected.user_requested,
         )
         if "PIP_RESOLVER_DEBUG" in os.environ:
-            reporter: BaseReporter = PipDebuggingReporter()
+            reporter: BaseReporter[Requirement, Candidate, Any] = PipDebuggingReporter()
         else:
             reporter = PipReporter()
         resolver: RLResolver[Requirement, Candidate, str] = RLResolver(
diff --git a/tests/unit/resolution_resolvelib/test_provider.py b/tests/unit/resolution_resolvelib/test_provider.py
index 5f30e2bc1dd..e3d43f6a3ef 100644
--- a/tests/unit/resolution_resolvelib/test_provider.py
+++ b/tests/unit/resolution_resolvelib/test_provider.py
@@ -5,6 +5,7 @@
 from pip._internal.models.candidate import InstallationCandidate
 from pip._internal.models.link import Link
 from pip._internal.req.constructors import install_req_from_req_string
+from pip._internal.resolution.resolvelib.base import Candidate
 from pip._internal.resolution.resolvelib.factory import Factory
 from pip._internal.resolution.resolvelib.provider import PipProvider
 from pip._internal.resolution.resolvelib.requirements import SpecifierRequirement
@@ -14,13 +15,13 @@
 
 
 def build_requirement_information(
-    name: str, parent: Optional[InstallationCandidate]
+    name: str, parent: Optional[Candidate]
 ) -> List["PreferenceInformation"]:
     install_requirement = install_req_from_req_string(name)
     # RequirementInformation is typed as a tuple, but it is a namedtupled.
     # https://github.com/sarugaku/resolvelib/blob/7bc025aa2a4e979597c438ad7b17d2e8a08a364e/src/resolvelib/resolvers.pyi#L20-L22
     requirement_information: PreferenceInformation = RequirementInformation(
-        requirement=SpecifierRequirement(install_requirement),  # type: ignore[call-arg]
+        requirement=SpecifierRequirement(install_requirement),
         parent=parent,
     )
     return [requirement_information]
@@ -60,7 +61,8 @@ def test_provider_known_depths(factory: Factory) -> None:
     transitive_requirement_name = "my-transitive-package"
 
     transitive_package_information = build_requirement_information(
-        name=transitive_requirement_name, parent=root_package_candidate
+        name=transitive_requirement_name,
+        parent=root_package_candidate,  # type: ignore
     )
     provider.get_preference(
         identifier=transitive_requirement_name,