Do not use TypeGuard context for lambda (#11417)

Closes #9927

Author: sobolevn

mypy/checkexpr.py

@@ -3560,6 +3560,12 @@ def infer_lambda_type_using_context(self, e: LambdaExpr) -> Tuple[Optional[Calla
         callable_ctx = get_proper_type(replace_meta_vars(ctx, ErasedType()))
         assert isinstance(callable_ctx, CallableType)
 
+        if callable_ctx.type_guard is not None:
+            # Lambda's return type cannot be treated as a `TypeGuard`,
+            # because it is implicit. And `TypeGuard`s must be explicit.
+            # See https://github.com/python/mypy/issues/9927
+            return None, None
+
         arg_kinds = [arg.kind for arg in e.arguments]
 
         if callable_ctx.is_ellipsis_args:

test-data/unit/check-typeguard.test

@@ -247,7 +247,7 @@ def main1(a: object) -> None:
 
 [builtins fixtures/tuple.pyi]
 
-[case testTypeGuardOverload-skip]
+[case testTypeGuardOverload]
 # flags: --strict-optional
 from typing import overload, Any, Callable, Iterable, Iterator, List, Optional, TypeVar
 from typing_extensions import TypeGuard