Allow dangerous identity comparisons to Any typed variables

Fixes #21134

Author: hauntsaninja

mypy/checkexpr.py

@@ -3800,8 +3800,20 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
             elif operator == "is" or operator == "is not":
                 right_type = self.accept(right)  # validate the right operand
                 sub_result = self.bool_type()
-                if not self.chk.can_skip_diagnostics and self.dangerous_comparison(
-                    left_type, right_type, identity_check=True
+                if (
+                    not self.chk.can_skip_diagnostics
+                    and self.dangerous_comparison(left_type, right_type, identity_check=True)
+                    # Allow dangerous identity comparisons with objects explicitly typed as Any
+                    and not (
+                        isinstance(left, NameExpr)
+                        and isinstance(left.node, Var)
+                        and isinstance(left.node.type, AnyType)
+                    )
+                    and not (
+                        isinstance(right, NameExpr)
+                        and isinstance(right.node, Var)
+                        and isinstance(right.node.type, AnyType)
+                    )
                 ):
                     # Show the most specific literal types possible
                     left_type = try_getting_literal(left_type)

test-data/unit/check-expressions.test

@@ -2380,6 +2380,40 @@ if 1 in ('x', 'y'):  # E: Non-overlapping container check (element type: "int",
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
 
+[case testStrictEqualityWithAnySentinel]
+# flags: --strict-equality
+from __future__ import annotations
+from typing import Any
+
+class A: ...
+class B: ...
+
+sentinel: Any = object()
+
+def f1(a: A = sentinel, b: B = sentinel):
+    if a is sentinel and b is sentinel:
+        raise
+
+
+def f2(a: A | None = sentinel, b: B | None = sentinel):
+    if a is sentinel and b is sentinel:
+        raise
+
+
+sentinel_strict = object()
+
+def f3(a: A = sentinel_strict, b: B = sentinel_strict):  # E: Incompatible default for parameter "a" (default has type "object", parameter has type "A") \
+                                                         # E: Incompatible default for parameter "b" (default has type "object", parameter has type "B")
+    if a is sentinel_strict and b is sentinel_strict:  # E: Non-overlapping identity check (left operand type: "B", right operand type: "A")
+        raise
+
+
+def f4(a: A | None = sentinel_strict, b: B | None = sentinel_strict):  # E: Incompatible default for parameter "a" (default has type "object", parameter has type "A | None") \
+                                                                       # E: Incompatible default for parameter "b" (default has type "object", parameter has type "B | None")
+    if a is sentinel_strict and b is sentinel_strict:  # E: Non-overlapping identity check (left operand type: "B | None", right operand type: "A | None")
+        raise
+[builtins fixtures/bool.pyi]
+
 [case testOverlappingAnyTypeWithoutStrictOptional]
 # flags: --no-strict-optional --strict-equality
 from typing import Any, Optional