[red-knot] Handle gradual intersection types in assignability

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

@@ -248,6 +248,7 @@ static_assert(is_assignable_to(Intersection[Child1, Parent], Parent))
 
 static_assert(is_assignable_to(Intersection[Parent, Unrelated], Parent))
 static_assert(is_assignable_to(Intersection[Child1, Unrelated], Child1))
+static_assert(is_assignable_to(Intersection[Child1, Unrelated, Child2], Intersection[Child1, Unrelated]))
 
 static_assert(is_assignable_to(Intersection[Child1, Not[Child2]], Child1))
 static_assert(is_assignable_to(Intersection[Child1, Not[Child2]], Parent))
@@ -256,21 +257,40 @@ static_assert(is_assignable_to(Intersection[Child1, Not[Grandchild]], Parent))
 static_assert(is_assignable_to(Intersection[Child1, Child2], Intersection[Child1, Child2]))
 static_assert(is_assignable_to(Intersection[Child1, Child2], Intersection[Child2, Child1]))
 static_assert(is_assignable_to(Grandchild, Intersection[Child1, Child2]))
+static_assert(not is_assignable_to(Intersection[Child1, Child2], Intersection[Parent, Unrelated]))
 
 static_assert(not is_assignable_to(Parent, Intersection[Parent, Unrelated]))
 static_assert(not is_assignable_to(int, Intersection[int, Not[Literal[1]]]))
+# The literal `1` is not assignable to `Parent`, so the intersection of int and Parent is definitely an int that is not `1`
+static_assert(is_assignable_to(Intersection[int, Parent], Intersection[int, Not[Literal[1]]]))
 static_assert(not is_assignable_to(int, Not[int]))
 static_assert(not is_assignable_to(int, Not[Literal[1]]))
 
-static_assert(not is_assignable_to(Intersection[Any, Parent], Unrelated))
-static_assert(is_assignable_to(Intersection[Unrelated, Any], Intersection[Unrelated, Any]))
+# Intersection with `Any` dominates the left hand side of intersections
+static_assert(is_assignable_to(Intersection[Any, Parent], Parent))
+static_assert(is_assignable_to(Intersection[Any, Child1], Parent))
+static_assert(is_assignable_to(Intersection[Any, Child2, Not[Child1]], Parent))
+static_assert(is_assignable_to(Intersection[Any, Parent], Unrelated))
+static_assert(is_assignable_to(Intersection[Any, Parent], Intersection[Parent, Unrelated]))
+static_assert(is_assignable_to(Intersection[Any, Parent, Unrelated], Parent))
+static_assert(is_assignable_to(Intersection[Any, Parent, Unrelated], Intersection[Parent, Unrelated]))
+
+# Even Any & Not[Parent] is assignable to Parent, since it could be Never
+static_assert(is_assignable_to(Intersection[Any, Not[Parent]], Parent))
+
+# Intersection with `Any` is effectively ignored on the right hand side for the sake of assignment
+static_assert(is_assignable_to(Parent, Intersection[Any, Parent]))
+static_assert(is_assignable_to(Parent, Parent | Intersection[Any, Unrelated]))
+static_assert(is_assignable_to(Child1, Intersection[Any, Parent]))
+static_assert(not is_assignable_to(Literal[1], Intersection[Any, Parent]))
+static_assert(not is_assignable_to(Unrelated, Intersection[Any, Parent]))
+
+# Intersections with Any on both sides combine the above logic - the LHS dominates and Any is ignored on the right hand side
+static_assert(is_assignable_to(Intersection[Any, Parent], Intersection[Any, Parent]))
+static_assert(is_assignable_to(Intersection[Any, Unrelated], Intersection[Any, Parent]))
+static_assert(is_assignable_to(Intersection[Any, Parent, Unrelated], Intersection[Any, Parent, Unrelated]))
 static_assert(is_assignable_to(Intersection[Unrelated, Any], Intersection[Unrelated, Not[Any]]))
-
-# TODO: The following assertions should not fail (see https://github.com/astral-sh/ruff/issues/14899)
-# error: [static-assert-error]
-static_assert(is_assignable_to(Intersection[Any, int], int))
-# error: [static-assert-error]
-static_assert(is_assignable_to(Intersection[Unrelated, Any], Not[tuple[Unrelated, Any]]))
+static_assert(is_assignable_to(Intersection[Literal[1], Any], Intersection[Unrelated, Not[Any]]))
 ```
 
 ## General properties

crates/red_knot_python_semantic/src/types.rs

@@ -802,6 +802,28 @@ impl<'db> Type<'db> {
                 .iter()
                 .any(|&elem_ty| ty.is_assignable_to(db, elem_ty)),
 
+            // A type S is assignable to an intersection type T if
+            // S is assignable to all positive elements of T (e.g. `str & int` is assignable to `str & Any`), and
+            // S is disjoint from all negative elements of T (e.g. `int` is not assignable to Intersection[int, Not[Literal[1]]]).
+            (ty, Type::Intersection(intersection)) => {
+                intersection
+                    .positive(db)
+                    .iter()
+                    .all(|&elem_ty| ty.is_assignable_to(db, elem_ty))
+                    && intersection
+                        .negative(db)
+                        .iter()
+                        .all(|&neg_ty| ty.is_disjoint_from(db, neg_ty))
+            }
+
+            // An intersection type S is assignable to a type T if
+            // Any element of S is assignable to T (e.g. `A & B` is assignable to `A`)
+            // Negative elements do not have an effect on assignability - if S is assignable to T then S & ~P is also assignable to T.
+            (Type::Intersection(intersection), ty) => intersection
+                .positive(db)
+                .iter()
+                .any(|&elem_ty| elem_ty.is_assignable_to(db, ty)),
+
             // A tuple type S is assignable to a tuple type T if their lengths are the same, and
             // each element of S is assignable to the corresponding element of T.
             (Type::Tuple(self_tuple), Type::Tuple(target_tuple)) => {

crates/ruff_benchmark/benches/red_knot.rs

@@ -67,28 +67,6 @@ static EXPECTED_DIAGNOSTICS: &[KeyDiagnosticFields] = &[
         Cow::Borrowed("Module `collections.abc` has no member `Iterable`"),
         Severity::Error,
     ),
-    // We don't handle intersections in `is_assignable_to` yet
-    (
-        DiagnosticId::lint("invalid-argument-type"),
-        Some("/src/tomllib/_parser.py"),
-        Some(20158..20172),
-        Cow::Borrowed("Object of type `Unknown & ~AlwaysFalsy | @Todo & ~AlwaysFalsy` cannot be assigned to parameter 1 (`match`) of function `match_to_datetime`; expected type `Match`"),
-        Severity::Error,
-    ),
-    (
-        DiagnosticId::lint("invalid-argument-type"),
-        Some("/src/tomllib/_parser.py"),
-        Some(20464..20479),
-        Cow::Borrowed("Object of type `Unknown & ~AlwaysFalsy | @Todo & ~AlwaysFalsy` cannot be assigned to parameter 1 (`match`) of function `match_to_localtime`; expected type `Match`"),
-        Severity::Error,
-    ),
-    (
-        DiagnosticId::lint("invalid-argument-type"),
-        Some("/src/tomllib/_parser.py"),
-        Some(20774..20786),
-        Cow::Borrowed("Object of type `Unknown & ~AlwaysFalsy | @Todo & ~AlwaysFalsy` cannot be assigned to parameter 1 (`match`) of function `match_to_number`; expected type `Match`"),
-        Severity::Error,
-    ),
     (
         DiagnosticId::lint("unused-ignore-comment"),
         Some("/src/tomllib/_parser.py"),