Honor NoReturn as __setitem__ return type to mark unreachable code.

Author: sterliakov

mypy/checker.py

@@ -3423,9 +3423,11 @@ def check_indexed_assignment(self, lvalue: IndexExpr,
             '__setitem__', basetype, lvalue)
 
         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) -> None:

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

@@ -1448,3 +1448,13 @@ a or a  # E: Right operand of "or" is never evaluated
 1 and a and 1  # E: Right operand of "and" is never evaluated
 a and a  # E: Right operand of "and" is never evaluated
 [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]