Add --strict-bytes to --strict (#19049)

This is a check that ensures static correctness, so it is useful to have
in --strict. Unlike making this the default behavior eventually in 2.0,
which we are also planning to do, it can be added to --strict
immediately due to --strict having looser backwards-compatibility
requirements (or so I interpret --strict's
[documentation](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-strict)).
This PR also includes tests, for --strict and also for no flags.

Author: wyattscarpenter

mypy/main.py

@@ -917,7 +917,7 @@ def add_invertible_flag(
     add_invertible_flag(
         "--strict-bytes",
         default=False,
-        strict_flag=False,
+        strict_flag=True,
         help="Disable treating bytearray and memoryview as subtypes of bytes",
         group=strictness_group,
     )

mypy_self_check.ini

@@ -1,7 +1,6 @@
 [mypy]
 
 strict = True
-strict_bytes = True
 local_partial_types = True
 disallow_any_unimported = True
 show_traceback = True

test-data/unit/check-flags.test

@@ -2408,6 +2408,23 @@ f(bytearray(b"asdf"))
 f(memoryview(b"asdf"))
 [builtins fixtures/primitives.pyi]
 
+[case testStrictBytesDisabledByDefault]
+# TODO: probably change this default in Mypy v2.0, with https://github.com/python/mypy/pull/18371
+# (this would also obsolete the testStrictBytesEnabledByStrict test, below)
+def f(x: bytes) -> None: ...
+f(bytearray(b"asdf"))
+f(memoryview(b"asdf"))
+[builtins fixtures/primitives.pyi]
+
+[case testStrictBytesEnabledByStrict]
+# flags: --strict --disable-error-code type-arg
+# The type-arg thing is just work around the primitives.pyi isinstance Tuple not having type parameters,
+#   which isn't important for this.
+def f(x: bytes) -> None: ...
+f(bytearray(b"asdf"))  # E: Argument 1 to "f" has incompatible type "bytearray"; expected "bytes"
+f(memoryview(b"asdf"))  # E: Argument 1 to "f" has incompatible type "memoryview"; expected "bytes"
+[builtins fixtures/primitives.pyi]
+
 [case testNoCrashFollowImportsForStubs]
 # flags: --config-file tmp/mypy.ini
 {**{"x": "y"}}