Avoid does not return error in lambda (#17294)

Fixes #10520, fixes #15142

Author: hauntsaninja

mypy/checker.py

@@ -4494,7 +4494,7 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
             is_lambda = isinstance(self.scope.top_function(), LambdaExpr)
             if isinstance(return_type, UninhabitedType):
                 # Avoid extra error messages for failed inference in lambdas
-                if not is_lambda or not return_type.ambiguous:
+                if not is_lambda and not return_type.ambiguous:
                     self.fail(message_registry.NO_RETURN_EXPECTED, s)
                     return
 

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

@@ -1502,15 +1502,8 @@ from typing import Callable, NoReturn
 def foo() -> NoReturn:
     raise
 
-f = lambda: foo()
+f1 = lambda: foo()
 x = 0  # not unreachable
 
-[case testLambdaNoReturnAnnotated]
-# flags: --warn-unreachable
-from typing import Callable, NoReturn
-
-def foo() -> NoReturn:
-    raise
-
-f: Callable[[], NoReturn] = lambda: foo()  # E: Return statement in function which does not return # (false positive: https://github.com/python/mypy/issues/17254)
+f2: Callable[[], NoReturn] = lambda: foo()
 x = 0  # not unreachable