Bug 905

ChangeLog

@@ -11,6 +11,14 @@ Release Date: TBA
 
   Closes #895 #899
 
+* Fix the `Duplicates found in MROs` false positive.
+
+  Closes #905
+  Closes PyCQA/pylint#4093 
+  Closes PyCQA/pylint#4131
+  Closes PyCQA/pylint#4145
+  Closes PyCQA/pylint#3247
+
 What's New in astroid 2.5?
 ============================
 Release Date: 2021-02-15

astroid/brain/brain_typing.py

@@ -11,9 +11,12 @@
     UseInferenceDefault,
     extract_node,
     inference_tip,
+    node_classes,
     nodes,
+    context,
     InferenceError,
 )
+import astroid
 
 
 TYPING_NAMEDTUPLE_BASENAMES = {"NamedTuple", "typing.NamedTuple"}
@@ -85,6 +88,48 @@ def infer_typing_attr(node, context=None):
     return node.infer(context=context)
 
 
+GET_ITEM_TEMPLATE = """
+@classmethod
+def __getitem__(cls, value):
+    return cls
+"""
+
+
+def _looks_like_typing_alias(node: nodes.Call) -> bool:
+    """
+    Returns True if the node corresponds to a call to _alias function.
+    For example :
+
+    MutableSet = _alias(collections.abc.MutableSet, T)
+
+    :param node: call node
+    """
+    if isinstance(node, nodes.Call) and isinstance(node.func, nodes.Name):
+        if node.func.name == "_alias" and isinstance(node.args[0], nodes.Attribute):
+            return True
+    return False
+
+
+def infer_typing_alias(
+    node: nodes.Call, context: context.InferenceContext = None
+) -> node_classes.NodeNG:
+    """
+    Infers the call to _alias function
+
+    :param node: call node
+    :param context: inference context
+    """
+    if not isinstance(node, nodes.Call):
+        return
+    res = next(node.args[0].infer(context=context))
+    #  Needs to mock the __getitem__ class method so that
+    #  MutableSet[T] is acceptable
+    func_to_add = extract_node(GET_ITEM_TEMPLATE)
+    if res.metaclass():
+        res.metaclass().locals["__getitem__"] = [func_to_add]
+    return res
+
+
 MANAGER.register_transform(
     nodes.Call,
     inference_tip(infer_typing_typevar_or_newtype),
@@ -93,3 +138,4 @@ def infer_typing_attr(node, context=None):
 MANAGER.register_transform(
     nodes.Subscript, inference_tip(infer_typing_attr), _looks_like_typing_subscript
 )
+MANAGER.register_transform(nodes.Call, infer_typing_alias, _looks_like_typing_alias)

tests/unittest_brain.py

@@ -1188,6 +1188,41 @@ def test_typing_namedtuple_dont_crash_on_no_fields(self):
         inferred = next(node.infer())
         self.assertIsInstance(inferred, astroid.Instance)
 
+    def test_typing_alias_type(self):
+        """
+        Test that the type aliased thanks to typing._alias function are
+        correctly inferred.
+        """
+        node = builder.extract_node(
+            """
+        from typing import TypeVar, MutableSet
+
+        T = TypeVar("T")
+        MutableSet[T]
+
+        class V(MutableSet[T]):
+            pass
+        """
+        )
+        inferred = next(node.infer())
+        mro_entries = list(inferred.mro())
+        self.assertIsInstance(mro_entries[0], astroid.ClassDef)
+        self.assertEqual(mro_entries[0].name, "V")
+        self.assertIsInstance(mro_entries[1], astroid.ClassDef)
+        self.assertEqual(mro_entries[1].name, "MutableSet")
+        self.assertIsInstance(mro_entries[2], astroid.ClassDef)
+        self.assertEqual(mro_entries[2].name, "Set")
+        self.assertIsInstance(mro_entries[3], astroid.ClassDef)
+        self.assertEqual(mro_entries[3].name, "Collection")
+        self.assertIsInstance(mro_entries[4], astroid.ClassDef)
+        self.assertEqual(mro_entries[4].name, "Sized")
+        self.assertIsInstance(mro_entries[5], astroid.ClassDef)
+        self.assertEqual(mro_entries[5].name, "Iterable")
+        self.assertIsInstance(mro_entries[6], astroid.ClassDef)
+        self.assertEqual(mro_entries[6].name, "Container")
+        self.assertIsInstance(mro_entries[7], astroid.ClassDef)
+        self.assertEqual(mro_entries[7].name, "object")
+
 
 class ReBrainTest(unittest.TestCase):
     def test_regex_flags(self):