Don't treat functions with reversible dunder names as such (#5421)

Previously, functions that had the same name as a reversible dunder
method (for example, __rpow__) were checked as if they were such a
method. This could lead to crashes and false positives.

Closes #5419

Author: srittau

mypy/checker.py

@@ -775,7 +775,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
                                            self.msg, context=fdef)
 
                 if name:  # Special method names
-                    if name in nodes.reverse_op_method_set:
+                    if defn.info and name in nodes.reverse_op_method_set:
                         self.check_reverse_op_method(item, typ, name, defn)
                     elif name in ('__getattr__', '__getattribute__'):
                         self.check_getattr_method(typ, defn, name)
@@ -976,6 +976,8 @@ def check_reverse_op_method(self, defn: FuncItem,
         # just decides whether it's worth calling
         # check_overlapping_op_methods().
 
+        assert defn.info is not None
+
         # First check for a valid signature
         method_type = CallableType([AnyType(TypeOfAny.special_form),
                                     AnyType(TypeOfAny.special_form)],

test-data/unit/check-modules.test

@@ -2495,3 +2495,12 @@ def __getattr__(attr: str) -> Any: ...
 class Cls: ...
 [builtins fixtures/module.pyi]
 [out]
+
+[case testFunctionWithDunderName]
+def __add__(self) -> int: ...
+
+[case testFunctionWithReversibleDunderName]
+def __radd__(self) -> int: ...
+
+[case testFunctionWithInPlaceDunderName]
+def __iadd__(self) -> int: ...