Modify infernce tip for typing.Generic and typing.Annotated with ``__class_getitem__`` (#931)

* Modify infernce tip for typing.Generic and typing.Annotated with __class_getitem__
* Fix issue with slots caching
* Clean typing.Generic from mro

5 files changed, 265 insertions, 9 deletions

diff --git a/ChangeLog b/ChangeLog
index 4b165767..a321d996 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -19,6 +19,12 @@ Release Date: TBA
 
 * Use ``inference_tip`` for ``typing.TypedDict`` brain.
 
+* Fix mro for classes that inherit from typing.Generic
+
+* Add inference tip for typing.Generic and typing.Annotated with ``__class_getitem__``
+
+  Closes PyCQA/pylint#2822
+
 
 What's New in astroid 2.5.2?
 ============================
diff --git a/astroid/brain/brain_typing.py b/astroid/brain/brain_typing.py
index 2e764465..9d94ca3e 100644
--- a/astroid/brain/brain_typing.py
+++ b/astroid/brain/brain_typing.py
@@ -88,6 +88,12 @@ TYPING_ALIAS = frozenset(
     )
 )
 
+CLASS_GETITEM_TEMPLATE = """
+@classmethod
+def __class_getitem__(cls, item):
+    return cls
+"""
+
 
 def looks_like_typing_typevar_or_newtype(node):
     func = node.func
@@ -126,7 +132,9 @@ def _looks_like_typing_subscript(node):
     return False
 
 
-def infer_typing_attr(node, context=None):
+def infer_typing_attr(
+    node: nodes.Subscript, ctx: context.InferenceContext = None
+) -> typing.Iterator[nodes.ClassDef]:
     """Infer a typing.X[...] subscript"""
     try:
         value = next(node.value.infer())
@@ -142,8 +150,31 @@ def infer_typing_attr(node, context=None):
         # (PY37+) handle it separately.
         raise UseInferenceDefault
 
+    if (
+        PY37
+        and isinstance(value, nodes.ClassDef)
+        and value.qname()
+        in ("typing.Generic", "typing.Annotated", "typing_extensions.Annotated")
+    ):
+        # With PY37+ typing.Generic and typing.Annotated (PY39) are subscriptable
+        # through __class_getitem__. Since astroid can't easily
+        # infer the native methods, replace them for an easy inference tip
+        func_to_add = astroid.extract_node(CLASS_GETITEM_TEMPLATE)
+        value.locals["__class_getitem__"] = [func_to_add]
+        if (
+            isinstance(node.parent, nodes.ClassDef)
+            and node in node.parent.bases
+            and getattr(node.parent, "__cache", None)
+        ):
+            # node.parent.slots is evaluated and cached before the inference tip
+            # is first applied. Remove the last result to allow a recalculation of slots
+            cache = getattr(node.parent, "__cache")
+            if cache.get(node.parent.slots) is not None:
+                del cache[node.parent.slots]
+        return iter([value])
+
     node = extract_node(TYPING_TYPE_TEMPLATE.format(value.qname().split(".")[-1]))
-    return node.infer(context=context)
+    return node.infer(context=ctx)
 
 
 def _looks_like_typedDict(  # pylint: disable=invalid-name
@@ -166,13 +197,6 @@ def infer_typedDict(  # pylint: disable=invalid-name
     return iter([class_def])
 
 
-CLASS_GETITEM_TEMPLATE = """
-@classmethod
-def __class_getitem__(cls, item):
-    return cls
-"""
-
-
 def _looks_like_typing_alias(node: nodes.Call) -> bool:
     """
     Returns True if the node corresponds to a call to _alias function.
diff --git a/astroid/scoped_nodes.py b/astroid/scoped_nodes.py
index dd5aa125..c3558e9a 100644
--- a/astroid/scoped_nodes.py
+++ b/astroid/scoped_nodes.py
@@ -103,6 +103,42 @@ def _c3_merge(sequences, cls, context):
     return None
 
 
+def clean_typing_generic_mro(sequences: List[List["ClassDef"]]) -> None:
+    """A class can inherit from typing.Generic directly, as base,
+    and as base of bases. The merged MRO must however only contain the last entry.
+    To prepare for _c3_merge, remove some typing.Generic entries from
+    sequences if multiple are present.
+
+    This method will check if Generic is in inferred_bases and also
+    part of bases_mro. If true, remove it from inferred_bases
+    as well as its entry the bases_mro.
+
+    Format sequences: [[self]] + bases_mro + [inferred_bases]
+    """
+    bases_mro = sequences[1:-1]
+    inferred_bases = sequences[-1]
+    # Check if Generic is part of inferred_bases
+    for i, base in enumerate(inferred_bases):
+        if base.qname() == "typing.Generic":
+            position_in_inferred_bases = i
+            break
+    else:
+        return
+    # Check if also part of bases_mro
+    # Ignore entry for typing.Generic
+    for i, seq in enumerate(bases_mro):
+        if i == position_in_inferred_bases:
+            continue
+        if any(base.qname() == "typing.Generic" for base in seq):
+            break
+    else:
+        return
+    # Found multiple Generics in mro, remove entry from inferred_bases
+    # and the corresponding one from bases_mro
+    inferred_bases.pop(position_in_inferred_bases)
+    bases_mro.pop(position_in_inferred_bases)
+
+
 def clean_duplicates_mro(sequences, cls, context):
     for sequence in sequences:
         names = [
@@ -2924,6 +2960,7 @@ class ClassDef(mixins.FilterStmtsMixin, LocalsDictNodeNG, node_classes.Statement
 
         unmerged_mro = [[self]] + bases_mro + [inferred_bases]
         unmerged_mro = list(clean_duplicates_mro(unmerged_mro, self, context))
+        clean_typing_generic_mro(unmerged_mro)
         return _c3_merge(unmerged_mro, self, context)
 
     def mro(self, context=None) -> List["ClassDef"]:
diff --git a/tests/unittest_brain.py b/tests/unittest_brain.py
index f15c4a1c..a217f223 100644
--- a/tests/unittest_brain.py
+++ b/tests/unittest_brain.py
@@ -1361,6 +1361,56 @@ class TypingBrain(unittest.TestCase):
             inferred = next(node.infer())
             self.assertIsInstance(inferred, nodes.ClassDef, node.as_string())
 
+    @test_utils.require_version(minver="3.7")
+    def test_typing_generic_subscriptable(self):
+        """Test typing.Generic is subscriptable with __class_getitem__ (added in PY37)"""
+        node = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        Generic[T]
+        """
+        )
+        inferred = next(node.infer())
+        assert isinstance(inferred, nodes.ClassDef)
+        assert isinstance(inferred.getattr("__class_getitem__")[0], nodes.FunctionDef)
+
+    @test_utils.require_version(minver="3.9")
+    def test_typing_annotated_subscriptable(self):
+        """Test typing.Annotated is subscriptable with __class_getitem__"""
+        node = builder.extract_node(
+            """
+        import typing
+        typing.Annotated[str, "data"]
+        """
+        )
+        inferred = next(node.infer())
+        assert isinstance(inferred, nodes.ClassDef)
+        assert isinstance(inferred.getattr("__class_getitem__")[0], nodes.FunctionDef)
+
+    @test_utils.require_version(minver="3.7")
+    def test_typing_generic_slots(self):
+        """Test cache reset for slots if Generic subscript is inferred."""
+        node = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A(Generic[T]):
+            __slots__ = ['value']
+            def __init__(self, value):
+                self.value = value
+        """
+        )
+        inferred = next(node.infer())
+        assert len(inferred.slots()) == 0
+        # Only after the subscript base is inferred and the inference tip applied,
+        # will slots contain the correct value
+        next(node.bases[0].infer())
+        slots = inferred.slots()
+        assert len(slots) == 1
+        assert isinstance(slots[0], nodes.Const)
+        assert slots[0].value == "value"
+
     def test_has_dunder_args(self):
         ast_node = builder.extract_node(
             """
diff --git a/tests/unittest_scoped_nodes.py b/tests/unittest_scoped_nodes.py
index a0f882ae..b98cd8f8 100644
--- a/tests/unittest_scoped_nodes.py
+++ b/tests/unittest_scoped_nodes.py
@@ -1275,6 +1275,9 @@ class ClassNodeTest(ModuleLoader, unittest.TestCase):
     def assertEqualMro(self, klass, expected_mro):
         self.assertEqual([member.name for member in klass.mro()], expected_mro)
 
+    def assertEqualMroQName(self, klass, expected_mro):
+        self.assertEqual([member.qname() for member in klass.mro()], expected_mro)
+
     @unittest.skipUnless(HAS_SIX, "These tests require the six library")
     def test_with_metaclass_mro(self):
         astroid = builder.parse(
@@ -1438,6 +1441,142 @@ class ClassNodeTest(ModuleLoader, unittest.TestCase):
         )
         self.assertEqualMro(cls, ["C", "A", "B", "object"])
 
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_1(self):
+        cls = builder.extract_node(
+            """
+        import typing
+        T = typing.TypeVar('T')
+        class A(typing.Generic[T]): ...
+        class B: ...
+        class C(A[T], B): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".C", ".A", "typing.Generic", ".B", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_2(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A: ...
+        class B(Generic[T]): ...
+        class C(Generic[T], A, B[T]): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".C", ".A", ".B", "typing.Generic", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_3(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A: ...
+        class B(A, Generic[T]): ...
+        class C(Generic[T]): ...
+        class D(B[T], C[T], Generic[T]): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".D", ".B", ".A", ".C", "typing.Generic", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_4(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A: ...
+        class B(Generic[T]): ...
+        class C(A, Generic[T], B[T]): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".C", ".A", ".B", "typing.Generic", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_5(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T1 = TypeVar('T1')
+        T2 = TypeVar('T2')
+        class A(Generic[T1]): ...
+        class B(Generic[T2]): ...
+        class C(A[T1], B[T2]): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".C", ".A", ".B", "typing.Generic", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_6(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic as TGeneric, TypeVar
+        T = TypeVar('T')
+        class Generic: ...
+        class A(Generic): ...
+        class B(TGeneric[T]): ...
+        class C(A, B[T]): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".C", ".A", ".Generic", ".B", "typing.Generic", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_7(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A(): ...
+        class B(Generic[T]): ...
+        class C(A, B[T]): ...
+        class D: ...
+        class E(C[str], D): ...
+        """
+        )
+        self.assertEqualMroQName(
+            cls, [".E", ".C", ".A", ".B", "typing.Generic", ".D", "builtins.object"]
+        )
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_error_1(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T1 = TypeVar('T1')
+        T2 = TypeVar('T2')
+        class A(Generic[T1], Generic[T2]): ...
+        """
+        )
+        with self.assertRaises(DuplicateBasesError) as ex:
+            cls.mro()
+
+    @test_utils.require_version(minver="3.7")
+    def test_mro_generic_error_2(self):
+        cls = builder.extract_node(
+            """
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class A(Generic[T]): ...
+        class B(A[T], A[T]): ...
+        """
+        )
+        with self.assertRaises(DuplicateBasesError) as ex:
+            cls.mro()
+
     def test_generator_from_infer_call_result_parent(self):
         func = builder.extract_node(
             """