Preserve original behavior for testLambdaReturningNone

Author: brianschubert

mypy/checkexpr.py

@@ -716,7 +716,9 @@ def always_returns_none(self, node: Expression) -> bool:
 
     def defn_returns_none(self, defn: SymbolNode | None) -> bool:
         """Check if `defn` can _only_ return None."""
+        allow_inferred = False
         if isinstance(defn, Decorator):
+            allow_inferred = True
             defn = defn.var
         if isinstance(defn, FuncDef):
             return isinstance(defn.type, CallableType) and isinstance(
@@ -726,8 +728,10 @@ def defn_returns_none(self, defn: SymbolNode | None) -> bool:
             return all(self.defn_returns_none(item) for item in defn.items)
         if isinstance(defn, Var):
             typ = get_proper_type(defn.type)
-            if isinstance(typ, CallableType) and isinstance(
-                get_proper_type(typ.ret_type), NoneType
+            if (
+                (allow_inferred or not defn.is_inferred)
+                and isinstance(typ, CallableType)
+                and isinstance(get_proper_type(typ.ret_type), NoneType)
             ):
                 return True
             if isinstance(typ, Instance):

test-data/unit/check-optional.test

@@ -119,8 +119,8 @@ reveal_type(z2)  # N: Revealed type is "Union[Literal[0], builtins.str, None]"
 
 [case testLambdaReturningNone]
 f = lambda: None
-x = f()         # E: Function does not return a value (it only ever returns None)
-reveal_type(x)  # N: Revealed type is "Any"
+x = f()
+reveal_type(x)  # N: Revealed type is "None"
 
 [case testNoneArgumentType]
 def f(x: None) -> None: pass