Always reset binder when checking deferred nodes (#17643)

Fixes #17595

Author: ilevkivskyi

mypy/checker.py

@@ -540,10 +540,11 @@ def check_partial(self, node: DeferredNodeType | FineGrainedDeferredNodeType) ->
             self.check_top_level(node)
         else:
             self.recurse_into_functions = True
-            if isinstance(node, LambdaExpr):
-                self.expr_checker.accept(node)
-            else:
-                self.accept(node)
+            with self.binder.top_frame_context():
+                if isinstance(node, LambdaExpr):
+                    self.expr_checker.accept(node)
+                else:
+                    self.accept(node)
 
     def check_top_level(self, node: MypyFile) -> None:
         """Check only the top-level of a module, skipping function definitions."""

test-data/unit/check-functions.test

@@ -3409,3 +3409,40 @@ for factory in (
     reveal_type(factory)  # N: Revealed type is "def () -> Union[builtins.str, None]"
     var = factory()
 [builtins fixtures/tuple.pyi]
+
+[case testLambdaInDeferredDecoratorNoCrash]
+def foo(x):
+    pass
+
+class Bar:
+    def baz(self, x):
+        pass
+
+class Qux(Bar):
+    @foo(lambda x: None)
+    def baz(self, x) -> None:
+        pass
+[builtins fixtures/tuple.pyi]
+
+[case testGeneratorInDeferredDecoratorNoCrash]
+from typing import Protocol, TypeVar
+
+T = TypeVar("T", covariant=True)
+
+class SupportsNext(Protocol[T]):
+    def __next__(self) -> T: ...
+
+def next(i: SupportsNext[T]) -> T: ...
+
+def foo(x):
+    pass
+
+class Bar:
+    def baz(self, x):
+        pass
+
+class Qux(Bar):
+    @next(f for f in [foo])
+    def baz(self, x) -> None:
+        pass
+[builtins fixtures/tuple.pyi]