Endless call cycle between is_callable_compatible and is_protocol_implementation

Discovered by @glyph and I see it's not his first rodeo (https://github.com/python/mypy/issues/18195 for instance).

My best interpretation (which could be wrong) is that a generic class which references itself in a method's type annotation, when typed using a protocol, will cause mypy to loop endlessly between matching the protocol and checking the types in the class. I haven't pinpointed the exact mechanism but it kind of makes sense that this could happen.

To Reproduce

The original code is here: https://github.com/glyph/OmniFocusStreamCounter/commit/454d0727363595b770cb150cc08a0018bc04a783

But this is about as far as I could reduce it:

from dataclasses import dataclass
from typing import Any, Callable, Protocol


class PExp[T](Protocol):
    def EQ(self, other: T) -> PExp[T]: ...


class PRef[T](Protocol):
    def EQ(self, other: T) -> PExp[T]: ...


@dataclass
class Exp[A, B]:
    _left: A
    _op: str
    _right: B

    def EQ(self, other: Any) -> Exp[Exp[A, B], Any]:
        return Exp(self, "EQ", other)


@dataclass
class Ref[T]:
    get: Callable[[], T]

    def EQ(self, other: T) -> Exp[Ref[T], T]:
        return Exp(self, "EQ", other)


@dataclass
class Cache:
    def spam(self, name: str) -> PRef[str]:
        return Ref(lambda: "spam")

Expected Behavior

So, if you make Exp.EQ untyped, then mypy will succeed (I fixed some problems in the original code 😉 ) This is obviously the expected behaviour.

Actual Behavior

Running mypy from HEAD, uncompiled, with --show-traceback - it hangs, then on Ctrl-C we see, this, endlessly:

  File "/home/dhd/work/mypy/mypy/subtypes.py", line 1281, in is_protocol_implementation                                                              07:44:34 [233/1846]
    is_compat = is_subtype(                                                                                                                                             
        subtype, supertype, ignore_pos_arg_names=ignore_names, options=options                                                                                          
    )                                                                                                                                                                   
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 189, in is_subtype                                                                                                  
    return _is_subtype(left, right, subtype_context, proper_subtype=False)                                                                                              
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 376, in _is_subtype                                                                                                 
    return left.accept(SubtypeVisitor(orig_right, subtype_context, proper_subtype))
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dhd/work/mypy/mypy/types.py", line 2361, in accept
    return visitor.visit_callable_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 740, in visit_callable_type
    return is_callable_compatible(
        left,
    ...<8 lines>...
        ),
    )
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 1785, in is_callable_compatible
    if not ignore_return and not is_compat_return(left.ret_type, right.ret_type):
                                 ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 448, in _is_subtype
    return is_subtype(left, right, subtype_context=self.subtype_context)
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 189, in is_subtype
    return _is_subtype(left, right, subtype_context, proper_subtype=False)
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 376, in _is_subtype
    return left.accept(SubtypeVisitor(orig_right, subtype_context, proper_subtype))
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dhd/work/mypy/mypy/types.py", line 1684, in accept
    return visitor.visit_instance(self)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/dhd/work/mypy/mypy/subtypes.py", line 627, in visit_instance
    if right.type.is_protocol and is_protocol_implementation(
                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~^
        left, right, proper_subtype=self.proper_subtype, options=self.options
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ):
    ^

Your Environment

• Mypy version used: mypy 2.4.0+dev.67eecf07fea3fa4bbaab9d46cd28372a8ded330b (compiled: no)
• Mypy command-line flags: --show-traceback -v
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
namespace_packages = true
plugins = ["mypy_zope:plugin"]

[[tool.mypy.overrides]]
ignore_missing_imports = true
module = [
  "objc",
  "Foundation",
  "AppKit",
  "ServiceManagement",
  "setuptools",
  "appscript",
]

• Python version used: 3.14

Start with the reduced Python reproducer and the traceback in mypy/subtypes.py, focusing on is_callable_compatible, is_protocol_implementation, and their calls through types.py. Reproduce the hang with --show-traceback -v, then trace the recursive subtype checks. Done means the example terminates with the expected result and regression coverage prevents the endless call cycle.