Exclude irrelevant members in narrow_declared_type from union overlapping with enum (#18897)

Fixes #18895.

The original implementation of that block was introduced as a
performance optimization in #12032. It's in fact incorrect: it produces
overly optimistic meets, assuming that *any* match among union items
makes them *all* relevant. As discussed in #18895, this actually results
in unexpected `meet` behaviour, as demonstrated by

```python
from enum import Enum
from typing_extensions import TypeIs, Literal

class Model(str, Enum):
    A = 'a'
    B = 'a'

def is_model_a(model: str) -> TypeIs[Literal[Model.A, "foo"]]:
    return True
def handle(model: Model) -> None:
    if is_model_a(model):
        reveal_type(model)  # N: Revealed type is "Union[Literal[__main__.Model.A], Literal['foo']]"


def is_int_or_list(model: object) -> TypeIs[int | list[int]]:
    return True
def compare(x: int | str) -> None:
    if is_int_or_list(x):
        reveal_type(x)  # N: Revealed type is "builtins.int"
```

This patch restores filtering of union members, but keeps it running
before the expensive `is_overlapping_types` check involving expansion.

Author: sterliakov

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 Literal, Union
+from typing_extensions import TypeGuard
+
+class Model(str, Enum):
+    A1 = 'model_a1'
+    A2 = 'model_a2'
+    B = 'model_b'
+
+MODEL_A = Literal[Model.A1, Model.A2]
+MODEL_B = Literal[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):
+        reveal_type(model)  # N: Revealed type is "__main__.Model"
+        return process_model(model)
+    return 0
+[builtins fixtures/tuple.pyi]

test-data/unit/check-typeis.test

@@ -936,3 +936,20 @@ def func(arg: Any) -> None:
     if is_dataclass(arg):
         reveal_type(arg)  # N: Revealed type is "Union[Type[__main__.DataclassInstance], __main__.DataclassInstance]"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeIsEnumOverlappingUnionExcludesIrrelevant]
+from enum import Enum
+from typing import Literal
+from typing_extensions import TypeIs
+
+class Model(str, Enum):
+    A = 'a'
+    B = 'a'
+
+def is_model_a(model: str) -> TypeIs[Literal[Model.A, "foo"]]:
+    return True
+
+def handle(model: Model) -> None:
+    if is_model_a(model):
+        reveal_type(model)  # N: Revealed type is "Literal[__main__.Model.A]"
+[builtins fixtures/tuple.pyi]