Only consider typevars same type after deeper comparison - we already have other places that create same-id copies of typevars

Author: sterliakov

mypy/subtypes.py

@@ -264,15 +264,17 @@ def is_same_type(
     a non-simplified union) but are semantically exchangeable in all contexts.
     """
     # First, use fast path for some common types. This is performance-critical.
-    if (
+    if a is b:
+        return True
+    elif (
         type(a) is Instance
         and type(b) is Instance
         and a.type == b.type
         and len(a.args) == len(b.args)
         and a.last_known_value is b.last_known_value
     ):
         return all(is_same_type(x, y) for x, y in zip(a.args, b.args))
-    elif isinstance(a, TypeVarType) and isinstance(b, TypeVarType) and a.id == b.id:
+    elif a == b:
         return True
 
     # Note that using ignore_promotions=True (default) makes types like int and int64

test-data/unit/check-expressions.test

@@ -731,6 +731,21 @@ def check(x: _T, y: _T) -> bool:
     return x < y
 [builtins fixtures/ops.pyi]
 
+[case testReversibleOpOnTypeVarProtocol]
+# https://github.com/python/mypy/issues/18203
+from typing import Protocol, TypeVar, Union
+from typing_extensions import Self
+
+class A(Protocol):
+    def __add__(self, other: Union[int, Self]) -> Self: ...
+    def __radd__(self, other: Union[int, Self]) -> Self: ...
+
+AT = TypeVar("AT", bound=Union[int, A])
+
+def f(a: AT, _b: AT) -> None:
+    a + a
+[builtins fixtures/ops.pyi]
+
 
 [case testErrorContextAndBinaryOperators]
 import typing