Account for non-fully-static TypeIs types in is_assignable_to()

Author: InSyncWithFoo

crates/ty_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -789,4 +789,17 @@ def g3(obj: Foo[tuple[A]]):
     f3(obj)
 ```
 
+## `TypeGuard` and `TypeIs`
+
+`TypeGuard[...]` and `TypeIs[...]` are always assignable to `bool`.
+
+```py
+from ty_extensions import Unknown, is_assignable_to, static_assert
+from typing_extensions import Any, TypeGuard, TypeIs
+
+# TODO: TypeGuard
+# static_assert(is_assignable_to(TypeGuard[Unknown], bool))
+static_assert(is_assignable_to(TypeIs[Any], bool))
+```
+
 [typing documentation]: https://typing.python.org/en/latest/spec/concepts.html#the-assignable-to-or-consistent-subtyping-relation

crates/ty_python_semantic/src/types.rs

@@ -1696,6 +1696,10 @@ impl<'db> Type<'db> {
             (Type::ProtocolInstance(_), _) => false,
             (_, Type::ProtocolInstance(protocol)) => self.satisfies_protocol(db, protocol),
 
+            (Type::TypeIs(_), _) => KnownClass::Bool
+                .to_instance(db)
+                .is_assignable_to(db, target),
+
             // TODO other types containing gradual forms
             _ => self.is_subtype_of(db, target),
         }