Fixes corner case for comparing nested overloads (python#9259)

sobolevn · web-flow · commit b10d5405efc3 · 2020-08-05T11:05:52.000-07:00
This is for a 3rd party plugin: dry-python/returns#410 - Closes python#9147 - Closes python#8978
diff --git a/mypy/subtypes.py b/mypy/subtypes.py
@@ -399,6 +399,10 @@ def visit_overloaded(self, left: Overloaded) -> bool:
                     return True
             return False
         elif isinstance(right, Overloaded):
+            if left == self.right:
+                # When it is the same overload, then the types are equal.
+                return True
+
             # Ensure each overload in the right side (the supertype) is accounted for.
             previous_match_left_index = -1
             matched_overloads = set()
