Always allow lambda calls (#17430)

See #17408 for context.

Author: ilevkivskyi

mypy/checkexpr.py

@@ -36,6 +36,7 @@
     ARG_STAR,
     ARG_STAR2,
     IMPLICITLY_ABSTRACT,
+    LAMBDA_NAME,
     LITERAL_TYPE,
     REVEAL_LOCALS,
     REVEAL_TYPE,
@@ -599,6 +600,7 @@ def visit_call_expr_inner(self, e: CallExpr, allow_none_return: bool = False) ->
             and self.chk.in_checked_function()
             and isinstance(callee_type, CallableType)
             and callee_type.implicit
+            and callee_type.name != LAMBDA_NAME
         ):
             if fullname is None and member is not None:
                 assert object_type is not None

mypy/nodes.py

@@ -175,6 +175,8 @@ def get_nongen_builtins(python_version: tuple[int, int]) -> dict[str, str]:
     "typing_extensions.runtime_checkable",
 )
 
+LAMBDA_NAME: Final = "<lambda>"
+
 
 class Node(Context):
     """Common base class for all non-type parse tree nodes."""
@@ -2262,7 +2264,7 @@ class LambdaExpr(FuncItem, Expression):
 
     @property
     def name(self) -> str:
-        return "<lambda>"
+        return LAMBDA_NAME
 
     def expr(self) -> Expression:
         """Return the expression (the body) of the lambda."""

mypy/plugins/functools.py

@@ -25,7 +25,7 @@
 
 _ORDERING_METHODS: Final = {"__lt__", "__le__", "__gt__", "__ge__"}
 
-PARTIAL = "functools.partial"
+PARTIAL: Final = "functools.partial"
 
 
 class _MethodInfo(NamedTuple):

test-data/unit/check-functions.test

@@ -3366,3 +3366,19 @@ class C(B):
     ) -> None:
         ...
 [builtins fixtures/tuple.pyi]
+
+[case testLambdaAlwaysAllowed]
+# flags: --disallow-untyped-calls
+from typing import Callable, Optional
+
+def func() -> Optional[str]: ...
+var: Optional[str]
+
+factory: Callable[[], Optional[str]]
+for factory in (
+    lambda: var,
+    func,
+):
+    reveal_type(factory)  # N: Revealed type is "def () -> Union[builtins.str, None]"
+    var = factory()
+[builtins fixtures/tuple.pyi]