Fix [func-returns-value] not being trigged when calling decorated functions

mypy/checkexpr.py

@@ -752,6 +752,10 @@ 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(
                 get_proper_type(defn.type.ret_type), NoneType
@@ -761,7 +765,7 @@ def defn_returns_none(self, defn: SymbolNode | None) -> bool:
         if isinstance(defn, Var):
             typ = get_proper_type(defn.type)
             if (
-                not defn.is_inferred
+                (allow_inferred or not defn.is_inferred)
                 and isinstance(typ, CallableType)
                 and isinstance(get_proper_type(typ.ret_type), NoneType)
             ):

test-data/unit/check-expressions.test

@@ -1140,6 +1140,52 @@ f() and b  # E: "f" does not return a value (it only ever returns None)
 b or f()   # E: "f" does not return a value (it only ever returns None)
 [builtins fixtures/bool.pyi]
 
+[case testNoneReturnDecorated]
+from typing import Any, Callable, TypeVar
+
+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 f1() -> None:
+    pass
+
+@deco
+@deco_return_none
+def f2() -> int:
+    pass
+
+@deco
+@deco_return_not_none
+def f3() -> None:
+    pass
+
+class A:
+    @staticmethod
+    def s() -> None:
+        pass
+
+if int():
+    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():
+    x = A().s()  # E: "s" of "A" does not return a value (it only ever returns None)
+[builtins fixtures/staticmethod.pyi]
+
 
 -- Slicing
 -- -------