Check signature of reverse operator methods (#4249)

Due to an old change, the signatures of reverse operator methods were not
checked. This change corrects that.

Fixes #4241.

Author: emmatyping

mypy/checker.py

@@ -641,7 +641,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
 
                 if name:  # Special method names
                     if name in nodes.reverse_op_method_set:
-                        self.check_reverse_op_method(item, typ, name)
+                        self.check_reverse_op_method(item, typ, name, defn)
                     elif name in ('__getattr__', '__getattribute__'):
                         self.check_getattr_method(typ, defn, name)
                     elif name == '__setattr__':
@@ -820,13 +820,24 @@ def is_trivial_body(self, block: Block) -> bool:
                  isinstance(stmt.expr, EllipsisExpr)))
 
     def check_reverse_op_method(self, defn: FuncItem, typ: CallableType,
-                                method: str) -> None:
+                                method: str, context: Context) -> None:
         """Check a reverse operator method such as __radd__."""
 
         # This used to check for some very obscure scenario.  It now
         # just decides whether it's worth calling
         # check_overlapping_op_methods().
 
+        # First check for a valid signature
+        method_type = CallableType([AnyType(TypeOfAny.special_form),
+                                    AnyType(TypeOfAny.special_form)],
+                                   [nodes.ARG_POS, nodes.ARG_POS],
+                                   [None, None],
+                                   AnyType(TypeOfAny.special_form),
+                                   self.named_type('builtins.function'))
+        if not is_subtype(typ, method_type):
+            self.msg.invalid_signature(typ, context)
+            return
+
         if method in ('__eq__', '__ne__'):
             # These are defined for all objects => can't cause trouble.
             return
@@ -839,9 +850,8 @@ def check_reverse_op_method(self, defn: FuncItem, typ: CallableType,
         if isinstance(ret_type, Instance):
             if ret_type.type.fullname() == 'builtins.object':
                 return
-        # Plausibly the method could have too few arguments, which would result
-        # in an error elsewhere.
-        if len(typ.arg_types) <= 2:
+
+        if len(typ.arg_types) == 2:
             # TODO check self argument kind
 
             # Check for the issue described above.

test-data/unit/check-classes.test

@@ -1639,6 +1639,18 @@ class C:
     def __radd__(self, x: Any) -> int: pass
 [out]
 
+[case testReverseOperatorMethodInvalid]
+from foo import *
+[file foo.pyi]
+class A: ...
+class B:
+    def __rmul__(self) -> A: ...
+class C:
+    def __radd__(self, other, oops) -> int: ...
+[out]
+tmp/foo.pyi:3: error: Invalid signature "def (foo.B) -> foo.A"
+tmp/foo.pyi:5: error: Invalid signature "def (foo.C, Any, Any) -> builtins.int"
+
 [case testReverseOperatorMethodForwardIsAny]
 from typing import Any
 def deco(f: Any) -> Any: return f