Fix indexed assignment check when TypedDict is used as upper bound (#8621)

Fixes #8576

Author: TH3CHARLie

mypy/checker.py

@@ -3049,8 +3049,15 @@ def check_indexed_assignment(self, lvalue: IndexExpr,
         """
         self.try_infer_partial_type_from_indexed_assignment(lvalue, rvalue)
         basetype = get_proper_type(self.expr_checker.accept(lvalue.base))
-        if isinstance(basetype, TypedDictType):
-            item_type = self.expr_checker.visit_typeddict_index_expr(basetype, lvalue.index)
+        if (isinstance(basetype, TypedDictType) or (isinstance(basetype, TypeVarType)
+                and isinstance(get_proper_type(basetype.upper_bound), TypedDictType))):
+            if isinstance(basetype, TypedDictType):
+                typed_dict_type = basetype
+            else:
+                upper_bound_type = get_proper_type(basetype.upper_bound)
+                assert isinstance(upper_bound_type, TypedDictType)
+                typed_dict_type = upper_bound_type
+            item_type = self.expr_checker.visit_typeddict_index_expr(typed_dict_type, lvalue.index)
             method_type = CallableType(
                 arg_types=[self.named_type('builtins.str'), item_type],
                 arg_kinds=[ARG_POS, ARG_POS],

test-data/unit/check-typeddict.test

@@ -2022,3 +2022,23 @@ class A:
 reveal_type(A().b)  # N: Revealed type is 'Any'
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]
+
+[case testTypedDictAsUpperBoundAndIndexedAssign]
+from typing import TypeVar, Generic, TypedDict
+
+
+class BaseDict(TypedDict, total=False):
+    foo: int
+
+
+_DICT_T = TypeVar('_DICT_T', bound=BaseDict)
+
+
+class SomeGeneric(Generic[_DICT_T]):
+    def __init__(self, data: _DICT_T) -> None:
+        self._data: _DICT_T = data
+
+    def set_state(self) -> None:
+        self._data['foo'] = 1
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-typeddict.pyi]