Exclude irrelevant members in narrow_declared_type from union overlapping with enum

mypy/meet.py

@@ -143,7 +143,12 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
             ]
         )
     if is_enum_overlapping_union(declared, narrowed):
-        return original_narrowed
+        # Quick check before reaching `is_overlapping_types`. If it's enum/literal overlap,
+        # avoid full expansion and make it faster.
+        assert isinstance(narrowed, UnionType)
+        return make_simplified_union(
+            [narrow_declared_type(declared, x) for x in narrowed.relevant_items()]
+        )
     elif not is_overlapping_types(declared, narrowed, prohibit_none_typevar_overlap=True):
         if state.strict_optional:
             return UninhabitedType()

test-data/unit/check-typeguard.test

@@ -803,3 +803,33 @@ def test() -> None:
         return
     reveal_type(x)  # N: Revealed type is "builtins.list[__main__.C]"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeGuardedTypeDoesNotLeak]
+# https://github.com/python/mypy/issues/18895
+from enum import Enum
+from typing import Union
+from typing_extensions import TypeGuard, Literal
+
+class Model(str, Enum):
+    MODEL_A1 = 'model_a1'
+    MODEL_A2 = 'model_a2'
+    MODEL_B = 'model_b'
+
+MODEL_A = Literal[Model.MODEL_A1, Model.MODEL_A2]
+MODEL_B = Literal[Model.MODEL_B]
+
+def is_model_a(model: str) -> TypeGuard[MODEL_A]:
+    return True
+
+def is_model_b(model: str) -> TypeGuard[MODEL_B]:
+    return True
+
+def process_model(model: Union[MODEL_A, MODEL_B]) -> int:
+    return 42
+
+def handle(model: Model) -> int:
+    if is_model_a(model) or is_model_b(model):
+        # TODO: should start passing after #18896
+        return process_model(model)  # E: Argument 1 to "process_model" has incompatible type "Model"; expected "Union[Literal[Model.MODEL_A1, Model.MODEL_A2], Literal[Model.MODEL_B]]"
+    return 0
+[builtins fixtures/tuple.pyi]