[red-knot] Empty tuple is always-falsy (#17213)

## Summary

Fix assignability of `tuple[()]` to `AlwaysFalsy`.

closes #17202 

## Test Plan

Ran the property tests for a while

Author: sharkdp

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

@@ -183,7 +183,7 @@ static_assert(is_assignable_to(Meta, type[Unknown]))
 ## Tuple types
 
 ```py
-from knot_extensions import static_assert, is_assignable_to
+from knot_extensions import static_assert, is_assignable_to, AlwaysTruthy, AlwaysFalsy
 from typing import Literal, Any
 
 static_assert(is_assignable_to(tuple[()], tuple[()]))
@@ -198,6 +198,14 @@ static_assert(is_assignable_to(tuple[()], tuple))
 static_assert(is_assignable_to(tuple[int, str], tuple))
 static_assert(is_assignable_to(tuple[Any], tuple))
 
+# TODO: It is not yet clear if we want the following two assertions to hold.
+# See https://github.com/astral-sh/ruff/issues/15528 for more details. The
+# short version is: We either need to special-case enforcement of the Liskov
+# substitution principle on `__bool__` and `__len__` for tuple subclasses,
+# or we need to negate these assertions.
+static_assert(is_assignable_to(tuple[()], AlwaysFalsy))
+static_assert(is_assignable_to(tuple[int], AlwaysTruthy))
+
 static_assert(not is_assignable_to(tuple[()], tuple[int]))
 static_assert(not is_assignable_to(tuple[int], tuple[str]))
 static_assert(not is_assignable_to(tuple[int], tuple[int, str]))

crates/red_knot_python_semantic/src/types.rs

@@ -1071,9 +1071,13 @@ impl<'db> Type<'db> {
             // This special case is required because the left-hand side tuple might be a
             // gradual type, so we can not rely on subtyping. This allows us to assign e.g.
             // `tuple[Any, int]` to `tuple`.
-            (Type::Tuple(_), _) => KnownClass::Tuple
-                .to_instance(db)
-                .is_assignable_to(db, target),
+            (Type::Tuple(_), _)
+                if KnownClass::Tuple
+                    .to_instance(db)
+                    .is_assignable_to(db, target) =>
+            {
+                true
+            }
 
             // `type[Any]` is assignable to any `type[...]` type, because `type[Any]` can
             // materialize to any `type[...]` type.