only abstract the typevars we actually depend on

crates/ty_python_semantic/src/types/constraints.rs

@@ -1042,11 +1042,11 @@ impl<'db> Node<'db> {
         db: &'db dyn Db,
         inferable: InferableTypeVars<'_, 'db>,
     ) -> bool {
-        match self {
+        let interior = match self {
             Node::AlwaysTrue => return true,
             Node::AlwaysFalse => return false,
-            Node::Interior(_) => {}
-        }
+            Node::Interior(interior) => interior,
+        };
 
         let mut typevars = FxHashSet::default();
         self.for_each_constraint(db, &mut |constraint| {
@@ -1059,8 +1059,6 @@ impl<'db> Node<'db> {
             }
         });
 
-        let without_inferable = self.exists(db, inferable.iter());
-
         // Returns if some specialization satisfies this constraint set.
         let some_specialization_satisfies = move |specializations: Node<'db>| {
             let when_satisfied = specializations.implies(db, self).and(db, specializations);
@@ -1068,9 +1066,10 @@ impl<'db> Node<'db> {
         };
 
         // Returns if all specializations satisfy this constraint set.
-        let all_specializations_satisfy = move |specializations: Node<'db>| {
+        let all_specializations_satisfy =
+            move |restricted: Node<'db>, specializations: Node<'db>| {
                 let when_satisfied = specializations
-                .implies(db, without_inferable)
+                    .implies(db, restricted)
                     .and(db, specializations);
                 when_satisfied
                     .iff(db, specializations)
@@ -1087,10 +1086,19 @@ impl<'db> Node<'db> {
                 }
             } else {
                 // If the typevar is in non-inferable position, we need to verify that all required
-                // specializations satisfy the constraint set. Complicating things, the typevar
-                // might have gradual constraints. For those, we need to know the range of valid
-                // materializations, but we only need some materialization to satisfy the
-                // constraint set.
+                // specializations satisfy the constraint set. If the typevar depends on any other
+                // typevars that are inferable, we are allowed to choose different specializations
+                // of those inferable typevars for each specialization of this non-inferable one.
+                // To handle this, we use the sequent map to find which inferable typevars this
+                // typevar depends on, and existentially abstract them away.
+                let inferable_dependencies = interior
+                    .sequent_map(db)
+                    .get_typevar_dependencies(typevar.identity(db));
+                let restricted = self.exists(db, inferable_dependencies);
+
+                // Complicating things, the typevar might have gradual constraints. For those, we
+                // need to know the range of valid materializations, but we only need some
+                // materialization to satisfy the constraint set.
                 //
                 // NB: We could also model this by introducing a synthetic typevar for the gradual
                 // constraint, treating that synthetic typevar as always inferable (so that we only
@@ -1099,7 +1107,7 @@ impl<'db> Node<'db> {
                 // constraint.
                 let (static_specializations, gradual_constraints) =
                     typevar.required_specializations(db);
-                if !all_specializations_satisfy(static_specializations) {
+                if !all_specializations_satisfy(restricted, static_specializations) {
                     return false;
                 }
                 for gradual_constraint in gradual_constraints {
@@ -2308,6 +2316,12 @@ struct SequentMap<'db> {
     >,
     /// Sequents of the form `C → D`
     single_implications: FxHashMap<ConstrainedTypeVar<'db>, FxHashSet<ConstrainedTypeVar<'db>>>,
+
+    /// A dependency map recording which typevars depend on each other. (A typevar `T` depends on a
+    /// typevar `U` if there is any constraint `T ≤ U` or `U ≤ T` in the constraint set.)
+    typevar_dependencies:
+        FxHashMap<BoundTypeVarIdentity<'db>, FxHashSet<BoundTypeVarIdentity<'db>>>,
+
     /// Constraints that we have already processed
     processed: FxHashSet<ConstrainedTypeVar<'db>>,
     /// Constraints that enqueued to be processed
@@ -2315,6 +2329,18 @@ struct SequentMap<'db> {
 }
 
 impl<'db> SequentMap<'db> {
+    fn get_typevar_dependencies(
+        &self,
+        bound_typevar: BoundTypeVarIdentity<'db>,
+    ) -> impl Iterator<Item = BoundTypeVarIdentity<'db>> + '_ {
+        self.typevar_dependencies
+            .get(&bound_typevar)
+            .map(|dependencies| dependencies.into_iter())
+            .into_iter()
+            .flatten()
+            .copied()
+    }
+
     fn add(&mut self, db: &'db dyn Db, constraint: ConstrainedTypeVar<'db>) {
         self.enqueue_constraint(constraint);
 
@@ -2404,6 +2430,22 @@ impl<'db> SequentMap<'db> {
             .insert(post);
     }
 
+    fn add_typevar_dependency(
+        &mut self,
+        left: BoundTypeVarIdentity<'db>,
+        right: BoundTypeVarIdentity<'db>,
+    ) {
+        // The typevar dependency map is reflexive, so add the dependency in both directions.
+        self.typevar_dependencies
+            .entry(left)
+            .or_default()
+            .insert(right);
+        self.typevar_dependencies
+            .entry(right)
+            .or_default()
+            .insert(left);
+    }
+
     fn add_sequents_for_single(&mut self, db: &'db dyn Db, constraint: ConstrainedTypeVar<'db>) {
         // If this constraint binds its typevar to `Never ≤ T ≤ object`, then the typevar can take
         // on any type, and the constraint is always satisfied.
@@ -2424,9 +2466,14 @@ impl<'db> SequentMap<'db> {
         // Technically, (1) also allows `(S = T) → (S = S)`, but the rhs of that is vacuously true,
         // so we don't add a sequent for that case.
 
+        let typevar = constraint.typevar(db);
+        let lower = constraint.lower(db);
+        let upper = constraint.upper(db);
         let post_constraint = match (lower, upper) {
             // Case 1
             (Type::TypeVar(lower_typevar), Type::TypeVar(upper_typevar)) => {
+                self.add_typevar_dependency(typevar.identity(db), lower_typevar.identity(db));
+                self.add_typevar_dependency(typevar.identity(db), upper_typevar.identity(db));
                 if !lower_typevar.is_same_typevar_as(db, upper_typevar) {
                     ConstrainedTypeVar::new(db, lower_typevar, Type::Never, upper)
                 } else {
@@ -2436,11 +2483,13 @@ impl<'db> SequentMap<'db> {
 
             // Case 2
             (Type::TypeVar(lower_typevar), _) => {
+                self.add_typevar_dependency(typevar.identity(db), lower_typevar.identity(db));
                 ConstrainedTypeVar::new(db, lower_typevar, Type::Never, upper)
             }
 
             // Case 3
             (_, Type::TypeVar(upper_typevar)) => {
+                self.add_typevar_dependency(typevar.identity(db), upper_typevar.identity(db));
                 ConstrainedTypeVar::new(db, upper_typevar, lower, Type::object())
             }