Convince Mypy we're doing the right thing

Author: uranusjr

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

@@ -33,6 +33,18 @@
 ]
 
 
+def as_base_candidate(candidate: Candidate) -> Optional[BaseCandidate]:
+    """The runtime version of BaseCandidate."""
+    base_candidate_classes = (
+        AlreadyInstalledCandidate,
+        EditableCandidate,
+        LinkCandidate,
+    )
+    if isinstance(candidate, base_candidate_classes):
+        return candidate
+    return None
+
+
 def make_install_req_from_link(link, template):
     # type: (Link, InstallRequirement) -> InstallRequirement
     assert not template.editable, "template is editable"

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

@@ -16,10 +16,8 @@
     cast,
 )
 
-from pip._vendor.packaging.requirements import (
-    InvalidRequirement,
-    Requirement as PackagingRequirement,
-)
+from pip._vendor.packaging.requirements import InvalidRequirement
+from pip._vendor.packaging.requirements import Requirement as PackagingRequirement
 from pip._vendor.packaging.specifiers import SpecifierSet
 from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
 from pip._vendor.pkg_resources import Distribution
@@ -57,6 +55,7 @@
     ExtrasCandidate,
     LinkCandidate,
     RequiresPythonCandidate,
+    as_base_candidate,
 )
 from .found_candidates import FoundCandidates, IndexCandidateInfo
 from .requirements import (
@@ -306,15 +305,19 @@ def find_candidates(
             identifier_req = PackagingRequirement(identifier)
         except InvalidRequirement:
             base_identifier = None
-            extras = frozenset()
+            extras: FrozenSet[str] = frozenset()
         else:
             base_identifier = identifier_req.name
             extras = frozenset(identifier_req.extras)
         if base_identifier and base_identifier in requirements:
             for req in requirements[base_identifier]:
-                base_cand, _ = req.get_candidate_lookup()
-                if base_cand is None:
+                lookup_cand, _ = req.get_candidate_lookup()
+                if lookup_cand is None:  # Not explicit.
                     continue
+                # We've stripped extras from the identifier, and should always
+                # get a BaseCandidate here, unless there's a bug elsewhere.
+                base_cand = as_base_candidate(lookup_cand)
+                assert base_cand is not None
                 candidate = self._make_extras_candidate(base_cand, extras)
                 explicit_candidates.add(candidate)