Fix narrowing for unions with Any

Previously we considered the else branch in the test case here
unreachable

Author: hauntsaninja

mypy/checker.py

@@ -8301,21 +8301,27 @@ def conditional_types(
             enum_name = target.fallback.type.fullname
             current_type = try_expanding_sum_type_to_union(current_type, enum_name)
 
+    proposed_type: Type | None
+    remaining_type: Type | None
+
     proper_type = get_proper_type(current_type)
     # factorize over union types: isinstance(A|B, C) -> yes = A_yes | B_yes
     if isinstance(proper_type, UnionType):
-        result: list[tuple[Type | None, Type | None]] = [
-            conditional_types(
+        yes_items: list[Type] = []
+        no_items: list[Type] = []
+        for union_item in proper_type.items:
+            yes_type, no_type = conditional_types(
                 union_item,
                 proposed_type_ranges,
                 default=union_item,
                 consider_runtime_isinstance=consider_runtime_isinstance,
             )
-            for union_item in get_proper_types(proper_type.items)
-        ]
-        # separate list of tuples into two lists
-        yes_types, no_types = zip(*result)
-        proposed_type = make_simplified_union([t for t in yes_types if t is not None])
+            yes_items.append(yes_type)
+            no_items.append(no_type)
+
+        proposed_type = make_simplified_union(yes_items)
+        remaining_type = make_simplified_union(no_items)
+        return proposed_type, remaining_type
     else:
         proposed_items = [type_range.item for type_range in proposed_type_ranges]
         proposed_type = make_simplified_union(proposed_items)

test-data/unit/check-narrowing.test

@@ -1608,6 +1608,21 @@ else:
 reveal_type(t)  # N: Revealed type is "Any"
 [builtins fixtures/isinstancelist.pyi]
 
+[case testNarrowingAnyUnion]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+from unknown import A, AA  # E: Cannot find implementation or library stub for module named "unknown" \
+                           # N: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+
+class B: ...
+
+def f(x: A | B) -> None:
+    if isinstance(x, (AA, B)):
+        reveal_type(x)  # N: Revealed type is "Any | __main__.B"
+    else:
+        reveal_type(x)  # N: Revealed type is "Any"
+[builtins fixtures/isinstancelist.pyi]
+
 [case testNarrowingLenItemAndLenCompare]
 # flags: --strict-equality --warn-unreachable
 from typing import Any