Reject unsafe operator method overrides

Author: JukkaL

mypy/checker.py

@@ -378,7 +378,17 @@ def check_override(self, override: FunctionLike, original: FunctionLike,
                     len(cast(Callable, original).arg_types) or
                 cast(Callable, override).min_args !=
                     cast(Callable, original).min_args):
+            # Use boolean variable to clarify code.
+            fail = False
             if not is_subtype(override, original):
+                fail = True
+            elif (not isinstance(original, Overloaded) and
+                  isinstance(override, Overloaded) and
+                  name in nodes.reverse_op_methods.keys()):
+                # Operator method overrides cannot introduce overloading, as
+                # this could be unsafe with reverse operator methods.
+                fail = True
+            if fail:
                 self.msg.signature_incompatible_with_supertype(
                     name, supertype, node)
             return

mypy/test/data/check-classes.test

@@ -828,6 +828,48 @@ main: In class "D":
 main, line 6: disjointclass constraint of class B disallows A as a base class
 
 
+-- Operator methods
+-- ----------------
+
+
+[case testOperatorMethodOverrideIntroducingOverloading]
+from typing import overload
+class A:
+    def __add__(self, x: int) -> int: pass
+class B(A):
+    @overload  # E: Signature of "__add__" incompatible with supertype "A"
+    def __add__(self, x: int) -> int: pass
+    @overload
+    def __add__(self, x: str) -> str: pass
+[out]
+main: In class "B":
+
+[case testOperatorMethodOverrideWideningArgumentType]
+from typing import overload
+class A:
+    def __add__(self, x: int) -> int: pass
+class B(A):
+    def __add__(self, x: object) -> int: pass # E: Argument 1 of "__add__" incompatible with supertype "A"
+[out]
+main: In class "B":
+
+[case testOperatorMethodOverrideNarrowingReturnType]
+from typing import overload
+class A:
+    def __add__(self, x: int) -> 'A': pass
+class B(A):
+    def __add__(self, x: int) -> 'B': pass
+[out]
+
+[case testOperatorMethodOverrideWithDynamicallyTyped]
+from typing import overload
+class A:
+    def __add__(self, x: int) -> 'A': pass
+class B(A):
+    def __add__(self, x): pass
+[out]
+
+
 -- Special cases
 -- -------------