[1.19 regression] Can no longer use `types.NoneType()` as a condition in match/case

[chadhpaine]

Bug Report

Prior to mypy 1.19, it was possible to use types.NoneType() as a pattern-matcher in match/case syntax for values whose 'revealed type' includes 'the None type'.

To Reproduce

import re
import types

def fun(val: str | re.Pattern | None):
 match val:
 case str():
 print('string')
 case re.Pattern():
 print('pattern')
 case types.NoneType(): # main.py:10: error: Expected type in class pattern; found "type[None]" [misc]
 print('None')

fun('123')
fun(re.compile('456'))
fun(None)

Expected Behavior

No error or warning is raised.

Actual Behavior

main.py:10: error: Expected type in class pattern; found "type[None]" [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Reproduces on mypy-play.net.
https://mypy-play.net/?mypy=latest&python=3.12&gist=3dc5c105d72ba997443f1c4f58a179a0

• Mypy version used: 1.19.0
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.12

[hauntsaninja]

Thanks, bisects to #20042

[hauntsaninja]

Something like this would fix, but probably want something a little more general

diff --git a/mypy/checkpattern.py b/mypy/checkpattern.py
index 3c51c4106..19a99f4e4 100644
--- a/mypy/checkpattern.py
+++ b/mypy/checkpattern.py
@@ -556,6 +557,8 @@ class PatternChecker(PatternVisitor[PatternType]):
 fallback = self.chk.named_type("builtins.function")
 any_type = AnyType(TypeOfAny.unannotated)
 typ = callable_with_ellipsis(any_type, ret_type=any_type, fallback=fallback)
+ elif isinstance(p_typ, TypeType) and isinstance(p_typ.item, NoneType):
+ typ = p_typ.item
 elif not isinstance(p_typ, AnyType):
 self.msg.fail(
 message_registry.CLASS_PATTERN_TYPE_REQUIRED.format(

[A5rocks]

I think that is the best we can do, unfortunately. type_info is a tempting variable to use, but doesn't work on type aliases.

Probably I should look at the logic in ExpressionChecker to see what other cases might be returned as a TypeType, but for now I'll just make a PR.