add ConstraintSetAssignability relation

crates/ty_python_semantic/src/types.rs

@@ -1955,6 +1955,33 @@ impl<'db> Type<'db> {
             return constraints.implies_subtype_of(db, self, target);
         }
 
+        // Handle the new constraint-set-based assignability relation next. Comparisons with a
+        // typevar are translated directly into a constraint set.
+        if relation.is_constraint_set_assignability() {
+            // A typevar satisfies a relation when...it satisfies the relation. Yes that's a
+            // tautology! We're moving the caller's subtyping/assignability requirement into a
+            // constraint set. If the typevar has an upper bound or constraints, then the relation
+            // only has to hold when the typevar has a valid specialization (i.e., one that
+            // satisfies the upper bound/constraints).
+            if let Type::TypeVar(bound_typevar) = self {
+                return ConstraintSet::constrain_typevar(
+                    db,
+                    bound_typevar,
+                    Type::Never,
+                    target,
+                    relation,
+                );
+            } else if let Type::TypeVar(bound_typevar) = target {
+                return ConstraintSet::constrain_typevar(
+                    db,
+                    bound_typevar,
+                    self,
+                    Type::object(),
+                    relation,
+                );
+            }
+        }
+
         match (self, target) {
             // Everything is a subtype of `object`.
             (_, Type::NominalInstance(instance)) if instance.is_object() => {
@@ -2035,7 +2062,7 @@ impl<'db> Type<'db> {
                 );
                 ConstraintSet::from(match relation {
                     TypeRelation::Subtyping | TypeRelation::SubtypingAssuming(_) => false,
-                    TypeRelation::Assignability => true,
+                    TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability => true,
                     TypeRelation::Redundancy => match target {
                         Type::Dynamic(_) => true,
                         Type::Union(union) => union.elements(db).iter().any(Type::is_dynamic),
@@ -2045,7 +2072,7 @@ impl<'db> Type<'db> {
             }
             (_, Type::Dynamic(_)) => ConstraintSet::from(match relation {
                 TypeRelation::Subtyping | TypeRelation::SubtypingAssuming(_) => false,
-                TypeRelation::Assignability => true,
+                TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability => true,
                 TypeRelation::Redundancy => match self {
                     Type::Dynamic(_) => true,
                     Type::Intersection(intersection) => {
@@ -2309,14 +2336,19 @@ impl<'db> Type<'db> {
                         TypeRelation::Subtyping
                         | TypeRelation::Redundancy
                         | TypeRelation::SubtypingAssuming(_) => self,
-                        TypeRelation::Assignability => self.bottom_materialization(db),
+                        TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability => {
+                            self.bottom_materialization(db)
+                        }
                     };
                     intersection.negative(db).iter().when_all(db, |&neg_ty| {
                         let neg_ty = match relation {
                             TypeRelation::Subtyping
                             | TypeRelation::Redundancy
                             | TypeRelation::SubtypingAssuming(_) => neg_ty,
-                            TypeRelation::Assignability => neg_ty.bottom_materialization(db),
+                            TypeRelation::Assignability
+                            | TypeRelation::ConstraintSetAssignability => {
+                                neg_ty.bottom_materialization(db)
+                            }
                         };
                         self_ty.is_disjoint_from_impl(
                             db,
@@ -11860,6 +11892,11 @@ pub(crate) enum TypeRelation<'db> {
     ///   are not actually subtypes of each other. (That is, `implies_subtype_of(false, int, str)`
     ///   will return true!)
     SubtypingAssuming(ConstraintSet<'db>),
+
+    /// A placeholder for the new assignability relation that uses constraint sets to encode
+    /// relationships with a typevar. This will eventually replace `Assignability`, but allows us
+    /// to start using the new relation in a controlled manner in some places.
+    ConstraintSetAssignability,
 }
 
 impl TypeRelation<'_> {
@@ -11867,6 +11904,10 @@ impl TypeRelation<'_> {
         matches!(self, TypeRelation::Assignability)
     }
 
+    pub(crate) const fn is_constraint_set_assignability(self) -> bool {
+        matches!(self, TypeRelation::ConstraintSetAssignability)
+    }
+
     pub(crate) const fn is_subtyping(self) -> bool {
         matches!(self, TypeRelation::Subtyping)
     }

crates/ty_python_semantic/src/types/class.rs

@@ -637,7 +637,9 @@ impl<'db> ClassType<'db> {
                     | TypeRelation::SubtypingAssuming(_) => {
                         ConstraintSet::from(other.is_object(db))
                     }
-                    TypeRelation::Assignability => ConstraintSet::from(!other.is_final(db)),
+                    TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability => {
+                        ConstraintSet::from(!other.is_final(db))
+                    }
                 },
 
                 // Protocol, Generic, and TypedDict are not represented by a ClassType.

crates/ty_python_semantic/src/types/constraints.rs

@@ -207,7 +207,7 @@ impl<'db> ConstraintSet<'db> {
                 lower.top_materialization(db),
                 upper.bottom_materialization(db),
             ),
-            TypeRelation::Assignability => (
+            TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability => (
                 lower.bottom_materialization(db),
                 upper.top_materialization(db),
             ),

crates/ty_python_semantic/src/types/generics.rs

@@ -903,7 +903,11 @@ fn has_relation_in_invariant_position<'db>(
             disjointness_visitor,
         ),
         // And A <~ B (assignability) is Bottom[A] <: Top[B]
-        (None, Some(base_mat), TypeRelation::Assignability) => is_subtype_in_invariant_position(
+        (
+            None,
+            Some(base_mat),
+            TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability,
+        ) => is_subtype_in_invariant_position(
             db,
             derived_type,
             MaterializationKind::Bottom,
@@ -913,7 +917,11 @@ fn has_relation_in_invariant_position<'db>(
             relation_visitor,
             disjointness_visitor,
         ),
-        (Some(derived_mat), None, TypeRelation::Assignability) => is_subtype_in_invariant_position(
+        (
+            Some(derived_mat),
+            None,
+            TypeRelation::Assignability | TypeRelation::ConstraintSetAssignability,
+        ) => is_subtype_in_invariant_position(
             db,
             derived_type,
             derived_mat,