Fix infinite recursion in protocol subtype check (GH#21739)

[mangodxd]

Description

Fixes #21739

Problem

When a generic class references itself in a method's return type via a protocol, mypy enters an infinite loop between is_callable_compatible and is_protocol_implementation.

The reproducer:

class PExp(Protocol):
    def EQ(self, other: Any) -> "PExp": ...

class PRef(Protocol):
    def EQ(self, other: Any) -> "PExp": ...

class Exp:
    def EQ(self, other: Any) -> "Exp":
        return Exp(self, "EQ", other)

class Ref:
    get: Callable[[], Any]
    def EQ(self, other: Any) -> "Exp":
        return Exp(self, "EQ", other)

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

Root Cause

The assuming mechanism in is_protocol_implementation tracks (left, right) pairs to detect recursive protocol checks. When it encounters the same pair again, it returns True (assuming compatibility).

However, in this case the left type keeps growing: Ref[T] → Exp[Ref[T], T] → Exp[Exp[Ref[T], T], Any] → ... The right protocol stays the same, but the exact-match check on l == left never triggers because each left is a different Instance type.

Fix

Add a guard to is_protocol_implementation: if the assuming stack exceeds 50 entries (meaning we're deep in recursive protocol checks for the same protocol), bail out assuming compatibility. This prevents infinite recursion while preserving correct behavior for all normal cases.

Tests

Added testProtocolInfiniteRecursionGrowingLeft in check-protocols.test using a simplified version of the reproducer from the issue.

[github-actions]

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