Fix handling of decorators that modify the function signature

Author: brianschubert

mypy/checkexpr.py

@@ -717,7 +717,7 @@ def always_returns_none(self, node: Expression) -> bool:
     def defn_returns_none(self, defn: SymbolNode | None) -> bool:
         """Check if `defn` can _only_ return None."""
         if isinstance(defn, Decorator):
-            defn = defn.func
+            defn = defn.var
         if isinstance(defn, FuncDef):
             return isinstance(defn.type, CallableType) and isinstance(
                 get_proper_type(defn.type.ret_type), NoneType
@@ -726,10 +726,8 @@ 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 (
-                not defn.is_inferred
-                and isinstance(typ, CallableType)
-                and isinstance(get_proper_type(typ.ret_type), NoneType)
+            if isinstance(typ, CallableType) and isinstance(
+                get_proper_type(typ.ret_type), NoneType
             ):
                 return True
             if isinstance(typ, Instance):

test-data/unit/check-expressions.test

@@ -1152,9 +1152,25 @@ F = TypeVar('F', bound=Callable[..., Any])
 def deco(f: F) -> F:
     pass
 
+def deco_return_none(f: object) -> Callable[..., None]:
+    pass
+
+def deco_return_not_none(f: object) -> Callable[..., int]:
+    pass
+
 @deco
 @deco
-def f() -> None:
+def f1() -> None:
+    pass
+
+@deco
+@deco_return_none
+def f2() -> int:
+    pass
+
+@deco
+@deco_return_not_none
+def f3() -> None:
     pass
 
 class A:
@@ -1163,7 +1179,11 @@ class A:
         pass
 
 if int():
-    x = f()      # E: "f" does not return a value (it only ever returns None)
+    x = f1()      # E: "f1" does not return a value (it only ever returns None)
+if int():
+    x = f2()      # E: "f2" does not return a value (it only ever returns None)
+if int():
+    x = f3()
 if int():
     x = A.s()    # E: "s" of "A" does not return a value (it only ever returns None)
 if int():