Handle expanding recursive protocol returns

[Rishav23av]

Fixes #21739.

The recursive protocol assumption stack currently closes only when the exact
candidate and protocol instances repeat. That does not happen when a callable
returns a strictly larger instance of its own generic class, so the subtype
check keeps alternating between callable compatibility and protocol
implementation until it raises RecursionError (or segfaults in a compiled
build).

This adds a deliberately narrow cycle closure for the safe case where:

• the exact protocol instance has already been seen with the same candidate class
• there is one non-settable, non-overloaded callable member
• both sides return themselves directly
• the candidate's non-return signature does not depend on its class type variables

Those conditions keep every non-return obligation unchanged while the candidate
return grows. Broader recursive shapes continue through the existing path.

The tests include the reported expanding-return case and a negative case where
the protocol changes from P[int] to P[object] before reaching a fixed point.
The latter ensures the new shortcut does not skip a changing argument
obligation.

Validation:

• python runtests.py self lint
• complete check-protocols.test: 202 passed, 1 skipped
• focused recursive-protocol group: 8 passed
• original reduced CLI reproduction: clean

Disclosure: I developed this change with assistance from Claude Code and Codex.
I reviewed the implementation, challenged several unsafe approaches with
counterexamples, and ran the checks listed above.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[glyph]

I don't think this fixes the issue. I upgraded via pip install 'git+https://github.com/Rishav23av/mypy@fix/21739-protocol-recursion' and then ran mypy --show-traceback ofchangewatcher.py on the same git revision and repo that #21739 suggests. I still get:

  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 1878, in is_callable_compatible
    unified = unify_generic_callable(left, right, ignore_return=ignore_return)
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 2221, in unify_generic_callable
    c = mypy.constraints.infer_constraints(
        type.ret_type, target.ret_type, return_constraint_direction
    )
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/constraints.py", line 320, in infer_constraints
    res = _infer_constraints(template, actual, direction, skip_neg_op, erase_types)
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/constraints.py", line 432, in _infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction, skip_neg_op, erase_types))
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 1684, in accept
    return visitor.visit_instance(self)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/constraints.py", line 973, in visit_instance
    and mypy.subtypes.is_protocol_implementation(erased, instance, skip=["__call__"])
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 1290, in is_protocol_implementation
    is_compat = is_subtype(
        subtype, supertype, ignore_pos_arg_names=ignore_names, options=options
    )
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 190, in is_subtype
    return _is_subtype(left, right, subtype_context, proper_subtype=False)
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 377, in _is_subtype
    return left.accept(SubtypeVisitor(orig_right, subtype_context, proper_subtype))
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 2361, in accept
    return visitor.visit_callable_type(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 741, in visit_callable_type
    return is_callable_compatible(
        left,
    ...<8 lines>...
        ),
    )
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 1878, in is_callable_compatible
    unified = unify_generic_callable(left, right, ignore_return=ignore_return)
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/subtypes.py", line 2221, in unify_generic_callable
    c = mypy.constraints.infer_constraints(
        type.ret_type, target.ret_type, return_constraint_direction
    )
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/constraints.py", line 307, in infer_constraints
    if any(
       ~~~^
        get_proper_type(template) == get_proper_type(t)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        and get_proper_type(actual) == get_proper_type(a)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        for (t, a) in reversed(type_state.inferring)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ):
    ^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/constraints.py", line 308, in <genexpr>
    get_proper_type(template) == get_proper_type(t)
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 1696, in __eq__
    and self.args == other.args
        ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 1696, in __eq__
    and self.args == other.args
        ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 1696, in __eq__
    and self.args == other.args
        ^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 262 more times]
  File "/Users/glyph/.virtualenvs/OmniFocusStreamCounter/lib/python3.13/site-packages/mypy/types.py", line 1691, in __eq__
    def __eq__(self, other: object) -> bool:
    
KeyboardInterrupt
Interrupted

[ilevkivskyi]

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