Add _value2member_map_ member to the enum brain. (#1820)

Refs pylint-dev/pylint#3941

ChangeLog

@@ -7,6 +7,10 @@ What's New in astroid 2.13.0?
 Release date: TBA
 
 
+* Add ``_value2member_map_`` member to the ``enum`` brain.
+
+  Refs PyCQA/pylint#3941
+
 
 What's New in astroid 2.12.11?
 ==============================

astroid/brain/brain_namedtuple_enum.py

@@ -425,6 +425,10 @@ def name(self):
                 new_targets.append(fake.instantiate_class())
                 dunder_members[local] = fake
             node.locals[local] = new_targets
+
+        # The undocumented `_value2member_map_` member:
+        node.locals["_value2member_map_"] = [nodes.Dict(parent=node)]
+
         members = nodes.Dict(parent=node)
         members.postinit(
             [

tests/unittest_scoped_nodes.py

@@ -2534,6 +2534,21 @@ class Veg(Enum):
     assert inferred_member_value.value is None
 
 
+def test_enums_value2member_map_() -> None:
+    """Check the `_value2member_map_` member is present in an Enum class"""
+    node = builder.extract_node(
+        """
+    from enum import Enum
+    class Veg(Enum):
+        TOMATO: 1
+
+    Veg
+    """
+    )
+    inferred_class = node.inferred()[0]
+    assert "_value2member_map_" in inferred_class.locals
+
+
 @pytest.mark.parametrize("annotation, value", [("int", 42), ("bytes", b"")])
 def test_enums_type_annotation_non_str_member(annotation, value) -> None:
     """A type-annotated member of an Enum class where: