Add type inference for dict.keys membership (#13372)

Closes #13360

Author: matthewhughes934

mypy/checker.py

@@ -6656,6 +6656,8 @@ def builtin_item_type(tp: Type) -> Type | None:
             "builtins.dict",
             "builtins.set",
             "builtins.frozenset",
+            "_collections_abc.dict_keys",
+            "typing.KeysView",
         ]:
             if not tp.args:
                 # TODO: fix tuple in lib-stub/builtins.pyi (it should be generic).

test-data/unit/pythoneval.test

@@ -1636,3 +1636,27 @@ foo("")
 foo(list(""))
 foo(list((list(""), "")))
 [out]
+
+[case testNarrowTypeForDictKeys]
+# flags: --strict-optional
+from typing import Dict, KeysView, Optional
+
+d: Dict[str, int]
+key: Optional[str]
+if key in d.keys():
+    reveal_type(key)
+else:
+    reveal_type(key)
+
+kv: KeysView[str]
+k: Optional[str]
+if k in kv:
+    reveal_type(k)
+else:
+    reveal_type(k)
+
+[out]
+_testNarrowTypeForDictKeys.py:7: note: Revealed type is "builtins.str"
+_testNarrowTypeForDictKeys.py:9: note: Revealed type is "Union[builtins.str, None]"
+_testNarrowTypeForDictKeys.py:14: note: Revealed type is "builtins.str"
+_testNarrowTypeForDictKeys.py:16: note: Revealed type is "Union[builtins.str, None]"