Introduce method to determine if decorator updates attributes (PY-27964)

python/src/com/jetbrains/python/inspections/unresolvedReference/PyUnresolvedReferencesInspection.java

@@ -826,7 +826,8 @@ public class PyUnresolvedReferencesInspection extends PyInspection {
       }
       if (type instanceof PyFunctionTypeImpl) {
         final PyCallable callable = ((PyFunctionTypeImpl)type).getCallable();
-        if (callable instanceof PyFunction && PyKnownDecoratorUtil.hasUnknownDecorator((PyFunction)callable, myTypeEvalContext)) {
+        if (callable instanceof PyFunction &&
+            PyKnownDecoratorUtil.hasUnknownOrUpdatingAttributesDecorator((PyFunction)callable, myTypeEvalContext)) {
           return true;
         }
       }

python/src/com/jetbrains/python/psi/PyKnownDecoratorUtil.java

@@ -219,6 +219,21 @@ public class PyKnownDecoratorUtil {
     return !allDecoratorsAreKnown(decoratable, decorators) || decorators.contains(UNITTEST_MOCK_PATCH);
   }
 
+  public static boolean hasUnknownOrUpdatingAttributesDecorator(@NotNull PyDecoratable decoratable, @NotNull TypeEvalContext context) {
+    final List<KnownDecorator> decorators = getKnownDecorators(decoratable, context);
+
+    if (!allDecoratorsAreKnown(decoratable, decorators)) {
+      return true;
+    }
+
+    return ContainerUtil.exists(
+      decorators,
+      d ->
+        d == FUNCTOOLS_LRU_CACHE || // cache_clear, cache_info
+        d == FUNCTOOLS_SINGLEDISPATCH // dispatch, register, registry
+    );
+  }
+
   private static boolean allDecoratorsAreKnown(@NotNull PyDecoratable element, @NotNull List<KnownDecorator> decorators) {
     final PyDecoratorList decoratorList = element.getDecoratorList();
     return decoratorList == null ? decorators.isEmpty() : decoratorList.getDecorators().length == decorators.size();

python/testData/inspections/PyUnresolvedReferencesInspection3K/usingFunctoolsSingledispatch.py

@@ -0,0 +1,16 @@
+from functools import singledispatch
+
+
+@singledispatch
+def to_description(ob):
+    return str(ob)
+
+
+@to_description.register(type(None))
+def none_to_description(_):
+    return '–'
+
+
+@to_description.register(bool)
+def bool_to_description(b):
+    return '✓' if b else ''

python/testSrc/com/jetbrains/python/inspections/Py3UnresolvedReferencesInspectionTest.java

@@ -265,4 +265,9 @@ public class Py3UnresolvedReferencesInspectionTest extends PyInspectionTestCase
   public void testNamespacePackageRedundantUnion() {
     doMultiFileTest("a.py", Arrays.asList("root1", "root2"));
   }
+
+  // PY-27964
+  public void testUsingFunctoolsSingledispatch() {
+    doTest();
+  }
 }