Avoid duplicate inference results for List[int]

This became necessary once we fixed the cache key in 0740a0d.

Author: jacobtylerwalls

astroid/brain/brain_typing.py

@@ -316,6 +316,9 @@ def infer_typing_alias(
         # This is an issue in cases where the aliased class implements it,
         # but the typing alias isn't subscriptable. E.g., `typing.ByteString` for PY39+
         _forbid_class_getitem_access(class_def)
+
+    # Avoid re-instantiating this class every time it's seen
+    node._explicit_inference = lambda node, context: iter([class_def])
     return iter([class_def])
 
 

tests/brain/test_brain.py

@@ -650,6 +650,16 @@ def __init__(self, value):
         assert isinstance(slots[0], nodes.Const)
         assert slots[0].value == "value"
 
+    @test_utils.require_version(minver="3.9")
+    def test_typing_no_duplicates(self):
+        node = builder.extract_node(
+            """
+        from typing import List
+        List[int]
+        """
+        )
+        assert len(node.inferred()) == 1
+
     def test_collections_generic_alias_slots(self):
         """Test slots for a class which is a subclass of a generic alias type."""
         node = builder.extract_node(