Allow overlapping comparisons between bytes-like types (#14658)

Author: hauntsaninja

mypy/checkexpr.py

@@ -192,6 +192,11 @@
     "_collections_abc.dict_keys",
     "_collections_abc.dict_items",
 ]
+OVERLAPPING_BYTES_ALLOWLIST: Final = {
+    "builtins.bytes",
+    "builtins.bytearray",
+    "builtins.memoryview",
+}
 
 
 class TooManyUnions(Exception):
@@ -3164,6 +3169,10 @@ def dangerous_comparison(
                 return self.dangerous_comparison(left.args[0], right.args[0])
             elif left_name in ("builtins.list", "builtins.tuple") and right_name == left_name:
                 return self.dangerous_comparison(left.args[0], right.args[0])
+            elif left_name in OVERLAPPING_BYTES_ALLOWLIST and right_name in (
+                OVERLAPPING_BYTES_ALLOWLIST
+            ):
+                return False
         if isinstance(left, LiteralType) and isinstance(right, LiteralType):
             if isinstance(left.value, bool) and isinstance(right.value, bool):
                 # Comparing different booleans is not dangerous.

test-data/unit/check-flags.test

@@ -2148,3 +2148,29 @@ def f(x: bytes) -> None: ...
 f(bytearray(b"asdf"))
 f(memoryview(b"asdf"))  # E: Argument 1 to "f" has incompatible type "memoryview"; expected "bytes"
 [builtins fixtures/primitives.pyi]
+
+[case testDisableBytearrayMemoryviewPromotionStrictEquality]
+# flags: --disable-bytearray-promotion --disable-memoryview-promotion --strict-equality
+def f(x: bytes, y: bytearray, z: memoryview) -> None:
+    x == y
+    y == z
+    x == z
+    97 in x
+    97 in y
+    97 in z
+    x in y
+    x in z
+[builtins fixtures/primitives.pyi]
+
+[case testEnableBytearrayMemoryviewPromotionStrictEquality]
+# flags: --strict-equality
+def f(x: bytes, y: bytearray, z: memoryview) -> None:
+    x == y
+    y == z
+    x == z
+    97 in x
+    97 in y
+    97 in z
+    x in y
+    x in z
+[builtins fixtures/primitives.pyi]