Add some assertions to BestCandidateResult.__init__().

src/pip/_internal/index.py

@@ -565,6 +565,13 @@ class BestCandidateResult(object):
         :param best_candidate: The most preferred candidate found, or None
             if no applicable candidates were found.
         """
+        assert set(applicable_candidates) <= set(candidates)
+
+        if best_candidate is None:
+            assert not applicable_candidates
+        else:
+            assert best_candidate in applicable_candidates
+
         self._applicable_candidates = applicable_candidates
         self._candidates = candidates