Reset lambda argument types in empty context

Author: ilevkivskyi

mypy/checkexpr.py

@@ -5591,6 +5591,10 @@ def visit_lambda_expr(self, e: LambdaExpr) -> Type:
                 self.chk.binder.frame_context(can_skip=True, fall_through=0),
                 self.chk.scope.push_function(e),
             ):
+                # If in empty context, reset argument types (from previous passes),
+                # in the other branch argument types are set by check_func_def().
+                for arg in e.arguments:
+                    arg.variable.type = None
                 # Lambdas can have more than one element in body,
                 # when we add "fictional" AssignmentStatement nodes, like in:
                 # `lambda (a, b): a`

test-data/unit/check-functions.test

@@ -2562,6 +2562,26 @@ reveal_type(f)
 [out]
 main:2: note: Revealed type is "def () -> builtins.int"
 
+[case testDeferredLambdaInOverload]
+from typing import Any, Callable, overload
+
+@overload
+def option(*, callback: Callable[[str], object] = ...) -> Any: ...
+@overload
+def option(*, callback: Callable[[int], object] = ...) -> Any: ...
+def option(**kwargs: object) -> None: pass
+
+def test() -> None:
+    option(callback=lambda x: [y for y in x])  # OK
+    x = C().x
+
+class C:
+    def __init__(self) -> None:
+        self.x = defer()
+
+def defer() -> int: ...
+[builtins fixtures/primitives.pyi]
+
 [case testRevealLocalsFunction]
 a = 1.0