Honor NoReturn as __setitem__ return type to mark unreachable code

Author: sterliakov

mypy/checker.py

@@ -4047,14 +4047,16 @@ def check_indexed_assignment(
         )
 
         lvalue.method_type = method_type
-        self.expr_checker.check_method_call(
+        res_type, _ = self.expr_checker.check_method_call(
             "__setitem__",
             basetype,
             method_type,
             [lvalue.index, rvalue],
             [nodes.ARG_POS, nodes.ARG_POS],
             context,
         )
+        if isinstance(get_proper_type(res_type), UninhabitedType):
+            self.binder.unreachable()
 
     def try_infer_partial_type_from_indexed_assignment(
         self, lvalue: IndexExpr, rvalue: Expression

test-data/unit/check-unreachable-code.test

@@ -1423,3 +1423,13 @@ def f(value: None) -> None:
 
 x = force_forward_ref()
 [builtins fixtures/exception.pyi]
+
+[case testSetitemNoReturn]
+# flags: --warn-unreachable
+from typing import NoReturn
+class Foo:
+    def __setitem__(self, key: str, value: str) -> NoReturn:
+        raise Exception
+Foo()['a'] = 'a'
+x = 0  # E: Statement is unreachable
+[builtins fixtures/exception.pyi]