[red-knot] is_assignable_to is not reflexive

[sharkdp]

The is_assignable_to-is-reflexive property test, which makes sure that we can assign any type to itself, is currently failing with counter-examples like Any & T, where T is an arbitrary type (that doesn't let the intersection collapse).

The reason for this is that we can't rely on equivalence for assignability of gradual types anymore ever since #14758. We do have some special casing for gradual types like Any and Unknown, as well as for unions and tuples of these, but we do lack handling of intersections.

This is not completely trivial. We can patch this with some superficial handling like

--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
@@ -748,6 +748,14 @@ impl<'db> Type<'db> {
                 .elements(db)
                 .iter()
                 .any(|&elem_ty| ty.is_assignable_to(db, elem_ty)),
+            (Type::Intersection(intersection), ty) => intersection
+                .positive(db)
+                .iter()
+                .any(|&elem_ty| elem_ty.is_assignable_to(db, ty)),
+            (ty, Type::Intersection(intersection)) => intersection
+                .positive(db)
+                .iter()
+                .any(|&elem_ty| ty.is_assignable_to(db, elem_ty)),
             (Type::Tuple(self_tuple), Type::Tuple(target_tuple)) => {
                 let self_elements = self_tuple.elements(db);
                 let target_elements = target_tuple.elements(db);

which also works for things like T & ~Any, because we always add Any/Unknown as positive contributions to intersections.

But then quickcheck finds types like ~tuple[Any, T], for which the simple handling above breaks.

[carljm]

Moving this out of Backlog and into Ready, and also marking help-wanted in case a contributor is interested in it, because I think having the quickcheck tests stable and green on main is quite valuable. Just now I was making another change to type relations, ran the quickcheck tests, and hit this failure, which diverted me for a while until I realized it was failing also on main and found this issue.