Add incremental tests for TypeGuard/TypeIs (#16976)

Author: JelleZijlstra

test-data/unit/check-incremental.test

@@ -6574,3 +6574,79 @@ class TheClass:
 [out]
 [out2]
 tmp/a.py:3: note: Revealed type is "def (value: builtins.object) -> lib.TheClass.pyenum@6"
+
+[case testStartUsingTypeGuard]
+import a
+[file a.py]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+
+[file a.py.2]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+if guard(x):
+    assert_type(x, int)
+else:
+    assert_type(x, Union[int, str])
+[file lib.py]
+from typing_extensions import TypeGuard
+def guard(x: object) -> TypeGuard[int]:
+    pass
+[builtins fixtures/tuple.pyi]
+
+[case testStartUsingTypeIs]
+import a
+[file a.py]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+
+[file a.py.2]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+if guard(x):
+    assert_type(x, int)
+else:
+    assert_type(x, str)
+[file lib.py]
+from typing_extensions import TypeIs
+def guard(x: object) -> TypeIs[int]:
+    pass
+[builtins fixtures/tuple.pyi]
+
+[case testTypeGuardToTypeIs]
+import a
+[file a.py]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+if guard(x):
+    assert_type(x, int)
+else:
+    assert_type(x, Union[int, str])
+[file a.py.2]
+from lib import guard
+from typing import Union
+from typing_extensions import assert_type
+x: Union[int, str]
+if guard(x):
+    assert_type(x, int)
+else:
+    assert_type(x, str)
+[file lib.py]
+from typing_extensions import TypeGuard
+def guard(x: object) -> TypeGuard[int]:
+    pass
+[file lib.py.2]
+from typing_extensions import TypeIs
+def guard(x: object) -> TypeIs[int]:
+    pass
+[builtins fixtures/tuple.pyi]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -68,7 +68,8 @@ class _TypedDict(Mapping[str, object]):
 
 def TypedDict(typename: str, fields: Dict[str, Type[_T]], *, total: Any = ...) -> Type[dict]: ...
 
-def reveal_type(__obj: T) -> T: pass
+def reveal_type(__obj: _T) -> _T: pass
+def assert_type(__val: _T, __typ: Any) -> _T: pass
 
 def dataclass_transform(
     *,
@@ -77,7 +78,7 @@ def dataclass_transform(
     kw_only_default: bool = ...,
     field_specifiers: tuple[type[Any] | Callable[..., Any], ...] = ...,
     **kwargs: Any,
-) -> Callable[[T], T]: ...
+) -> Callable[[_T], _T]: ...
 
 def override(__arg: _T) -> _T: ...
 def deprecated(__msg: str) -> Callable[[_T], _T]: ...