Add functional tests for narrow_requirement_selection

tests/functional/python/test_resolvers_python.py

@@ -121,6 +121,24 @@ def get_dependencies(self, candidate):
         return list(self._iter_dependencies(candidate))
 
 
+class PythonInputProviderNarrowRequirements(PythonInputProvider):
+    def narrow_requirement_selection(
+        self, identifiers, resolutions, candidates, information, backtrack_causes
+    ):
+        # Consider requirements that have 0 candidates (a resolution end point
+        # that can be backtracked from) or 1 candidate (speeds up situations where
+        # ever requirement is pinned to 1 specific version)
+        number_of_candidates = defaultdict(list)
+        for identifier in identifiers:
+            number_of_candidates[len(list(candidates[identifier]))].append(identifier)
+
+        min_candidates = min(number_of_candidates.keys())
+        if min_candidates in (0, 1):
+            return number_of_candidates[min_candidates]
+
+        return identifiers
+
+
 INPUTS_DIR = os.path.abspath(os.path.join(__file__, "..", "inputs"))
 
 CASE_DIR = os.path.join(INPUTS_DIR, "case")
@@ -133,20 +151,32 @@ def get_dependencies(self, candidate):
 }
 
 
-@pytest.fixture(
-    params=[
+def create_params(provider_class):
+    return [
         pytest.param(
-            os.path.join(CASE_DIR, n),
+            (os.path.join(CASE_DIR, n), provider_class),
             marks=pytest.mark.xfail(strict=True, reason=XFAIL_CASES[n]),
         )
         if n in XFAIL_CASES
-        else os.path.join(CASE_DIR, n)
+        else (os.path.join(CASE_DIR, n), provider_class)
+        for n in CASE_NAMES
+    ]
+
+
+@pytest.fixture(
+    params=[
+        *create_params(PythonInputProvider),
+        *create_params(PythonInputProviderNarrowRequirements),
+    ],
+    ids=[
+        f"{n[:-5]}-{cls.__name__}"
+        for cls in [PythonInputProvider, PythonInputProviderNarrowRequirements]
         for n in CASE_NAMES
     ],
-    ids=[n[:-5] for n in CASE_NAMES],
 )
 def provider(request):
-    return PythonInputProvider(request.param)
+    path, provider_class = request.param
+    return provider_class(path)
 
 
 def _format_confliction(exception):