[ty] Simple subtyping support for bidirectional inference

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -542,8 +542,7 @@ e: list[Any] | None = [1]
 reveal_type(e)  # revealed: list[Any]
 
 f: list[Any] | None = f2(1)
-# TODO: Better constraint solver.
-reveal_type(f)  # revealed: list[int] | None
+reveal_type(f)  # revealed: list[Any] | None
 
 g: list[Any] | dict[Any, Any] = f3(1)
 # TODO: Better constraint solver.
@@ -600,6 +599,48 @@ reveal_type(x7)  # revealed: Contravariant[Any]
 reveal_type(x8)  # revealed: Invariant[Any]
 ```
 
+## Declared type preference sees through subtyping
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import Any, Iterable, Literal, MutableSequence, Sequence
+
+x1: Sequence[Any] = [1, 2, 3]
+reveal_type(x1)  # revealed: list[Any]
+
+x2: MutableSequence[Any] = [1, 2, 3]
+reveal_type(x2)  # revealed: list[Any]
+
+x3: Iterable[Any] = [1, 2, 3]
+reveal_type(x3)  # revealed: list[Any]
+
+class X[T]:
+    value: T
+
+    def __init__(self, value: T): ...
+
+class A[T](X[T]): ...
+
+def a[T](value: T) -> A[T]:
+    return A(value)
+
+x4: A[object] = A(1)
+reveal_type(x4)  # revealed: A[object]
+
+x5: X[object] = A(1)
+reveal_type(x5)  # revealed: A[object]
+
+x6: X[object] | None = A(1)
+reveal_type(x6)  # revealed: A[object]
+
+x7: X[object] | None = a(1)
+reveal_type(x7)  # revealed: A[object]
+```
+
 ## Narrow generic unions
 
 ```toml

crates/ty_python_semantic/resources/mdtest/dataclasses/fields.md

@@ -37,7 +37,7 @@ class Data:
     content: list[int] = field(default_factory=list)
     timestamp: datetime = field(default_factory=datetime.now, init=False)
 
-# revealed: (self: Data, content: list[int] = list[int]) -> None
+# revealed: (self: Data, content: list[int] = Unknown) -> None
 reveal_type(Data.__init__)
 
 data = Data([1, 2, 3])

crates/ty_python_semantic/resources/mdtest/literal_promotion.md

@@ -341,3 +341,52 @@ reveal_type(x21)  # revealed: X[Literal[1]]
 x22: X[Literal[1]] | None = x(1)
 reveal_type(x22)  # revealed: X[Literal[1]]
 ```
+
+## Literal annotations see through subtyping
+
+```py
+from typing import Any, Iterable, Literal, MutableSequence, Sequence
+
+x1: Sequence[Literal[1, 2, 3]] = [1, 2, 3]
+reveal_type(x1)  # revealed: list[Literal[1, 2, 3]]
+
+x2: MutableSequence[Literal[1, 2, 3]] = [1, 2, 3]
+reveal_type(x2)  # revealed: list[Literal[1, 2, 3]]
+
+x3: Iterable[Literal[1, 2, 3]] = [1, 2, 3]
+reveal_type(x3)  # revealed: list[Literal[1, 2, 3]]
+
+class Sup1[T]:
+    value: T
+
+class Sub1[T](Sup1[T]): ...
+
+def sub1[T](value: T) -> Sub1[T]:
+    return Sub1()
+
+x4: Sub1[Literal[1]] = sub1(1)
+reveal_type(x4)  # revealed: Sub1[Literal[1]]
+
+x5: Sup1[Literal[1]] = sub1(1)
+reveal_type(x5)  # revealed: Sub1[Literal[1]]
+
+x6: Sup1[Literal[1]] | None = sub1(1)
+reveal_type(x6)  # revealed: Sub1[Literal[1]]
+
+x7: Sup1[Literal[1]] | None = sub1(1)
+reveal_type(x7)  # revealed: Sub1[Literal[1]]
+
+class Sup2[T, U]:
+    value: tuple[T, U]
+
+class Sub2[T, U](Sup2[T, Any], Sup2[Any, U]): ...
+
+def sub2[T, U](x: T, y: U) -> Sub2[T, U]:
+    return Sub2()
+
+x8 = sub2(1, 2)
+reveal_type(x8)  # revealed: Sub2[int, int]
+
+x9: Sup2[Literal[1], Literal[2]] = sub2(1, 2)
+reveal_type(x9)  # revealed: Sub2[Literal[1], Literal[2]]
+```

crates/ty_python_semantic/resources/mdtest/type_compendium/tuple.md

@@ -57,6 +57,9 @@ reveal_type(tuple((1, 2)))  # revealed: tuple[Literal[1], Literal[2]]
 
 reveal_type(tuple([1]))  # revealed: tuple[Unknown | int, ...]
 
+x1: tuple[int, ...] = tuple([1])
+reveal_type(x1)  # revealed: tuple[int, ...]
+
 # error: [invalid-argument-type]
 reveal_type(tuple[int]([1]))  # revealed: tuple[int]
 

crates/ty_python_semantic/src/types.rs

@@ -2,6 +2,7 @@ use compact_str::{CompactString, ToCompactString};
 use infer::nearest_enclosing_class;
 use itertools::{Either, Itertools};
 use ruff_diagnostics::{Edit, Fix};
+use rustc_hash::FxHashSet;
 
 use std::borrow::Cow;
 use std::time::Duration;
@@ -1000,7 +1001,10 @@ impl<'db> Type<'db> {
     }
 
     /// If this type is a class instance, returns its specialization.
-    pub(crate) fn class_specialization(self, db: &'db dyn Db) -> Option<Specialization<'db>> {
+    pub(crate) fn class_specialization(
+        self,
+        db: &'db dyn Db,
+    ) -> Option<(ClassType<'db>, Specialization<'db>)> {
         self.specialization_of_optional(db, None)
     }
 
@@ -1011,15 +1015,17 @@ impl<'db> Type<'db> {
         expected_class: ClassLiteral<'_>,
     ) -> Option<Specialization<'db>> {
         self.specialization_of_optional(db, Some(expected_class))
+            .map(|(_, specialization)| specialization)
     }
 
     fn specialization_of_optional(
         self,
         db: &'db dyn Db,
         expected_class: Option<ClassLiteral<'_>>,
-    ) -> Option<Specialization<'db>> {
+    ) -> Option<(ClassType<'db>, Specialization<'db>)> {
         let class_type = match self {
             Type::NominalInstance(instance) => instance,
+            Type::ProtocolInstance(instance) => instance.to_nominal_instance()?,
             Type::TypeAlias(alias) => alias.value_type(db).as_nominal_instance()?,
             _ => return None,
         }
@@ -1030,7 +1036,64 @@ impl<'db> Type<'db> {
             return None;
         }
 
-        specialization
+        Some((class_type, specialization?))
+    }
+
+    /// Given a type variable `T` from the generic context of a class `C`:
+    /// - If `self` is a specialized instance of `C`, returns the type assigned to `T` on `self`.
+    /// - If `self` is a specialized instance of some class `A`, and `C` is a subclass of `A`
+    ///   such that the type variable `U` on `A` is specialized to `T`, returns the type
+    ///   assigned to `U` on `self`.
+    pub(crate) fn find_type_var_from(
+        self,
+        db: &'db dyn Db,
+        bound_typevar: BoundTypeVarInstance<'db>,
+        class: ClassLiteral<'db>,
+    ) -> Option<Type<'db>> {
+        self.find_type_var_from_impl(db, bound_typevar, class, &mut FxHashSet::default())
+    }
+
+    pub(crate) fn find_type_var_from_impl(
+        self,
+        db: &'db dyn Db,
+        bound_typevar: BoundTypeVarInstance<'db>,
+        class: ClassLiteral<'db>,
+        visited: &mut FxHashSet<(ClassLiteral<'db>, BoundTypeVarIdentity<'db>)>,
+    ) -> Option<Type<'db>> {
+        if let Some(specialization) = self.specialization_of(db, class) {
+            return specialization.get(db, bound_typevar);
+        }
+
+        // TODO: We should use the constraint solver here to determine the type mappings for more
+        // complex subtyping relationships, e.g., `type[C[T]]` to `Callable[..., T]`, or unions
+        // containing multiple generic elements.
+        for base in class.iter_mro(db, None) {
+            let Some((origin, Some(specialization))) =
+                base.into_class().map(|class| class.class_literal(db))
+            else {
+                continue;
+            };
+
+            for (base_typevar, base_ty) in specialization
+                .generic_context(db)
+                .variables(db)
+                .zip(specialization.types(db))
+            {
+                if *base_ty == Type::TypeVar(bound_typevar) {
+                    if !visited.insert((origin, base_typevar.identity(db))) {
+                        return None;
+                    }
+
+                    if let Some(ty) =
+                        self.find_type_var_from_impl(db, base_typevar, origin, visited)
+                    {
+                        return Some(ty);
+                    }
+                }
+            }
+        }
+
+        None
     }
 
     /// Returns the top materialization (or upper bound materialization) of this type, which is the
@@ -3852,20 +3915,20 @@ impl<'db> Type<'db> {
             return;
         };
 
-        let tcx_specialization = tcx.annotation.and_then(|tcx| {
-            tcx.filter_union(db, |ty| ty.specialization_of(db, class_literal).is_some())
-                .specialization_of(db, class_literal)
-        });
-
-        for (typevar, ty) in specialization
+        for (type_var, ty) in specialization
             .generic_context(db)
             .variables(db)
             .zip(specialization.types(db))
         {
-            let variance = typevar.variance_with_polarity(db, polarity);
-            let tcx = TypeContext::new(tcx_specialization.and_then(|spec| spec.get(db, typevar)));
+            let variance = type_var.variance_with_polarity(db, polarity);
+            let tcx = tcx.and_then(|tcx| {
+                tcx.filter_union(db, |ty| {
+                    ty.find_type_var_from(db, type_var, class_literal).is_some()
+                })
+                .find_type_var_from(db, type_var, class_literal)
+            });
 
-            f(typevar, *ty, variance, tcx);
+            f(type_var, *ty, variance, tcx);
 
             visitor.visit(*ty, || {
                 ty.visit_specialization_impl(db, tcx, variance, f, visitor);
@@ -6059,30 +6122,35 @@ impl<'db> Type<'db> {
                 }
 
                 Some(KnownClass::Tuple) => {
-                    let object = Type::object();
+                    let element_ty =
+                        BoundTypeVarInstance::synthetic(db, "T", TypeVarVariance::Covariant);
 
                     // ```py
-                    // class tuple:
+                    // class tuple(Sequence[_T_co]):
                     //     @overload
                     //     def __new__(cls) -> tuple[()]: ...
                     //     @overload
-                    //     def __new__(cls, iterable: Iterable[object]) -> tuple[object, ...]: ...
+                    //     def __new__(cls, iterable: Iterable[_T_co]) -> tuple[_T_co, ...]: ...
                     // ```
                     CallableBinding::from_overloads(
                         self,
                         [
                             Signature::new(Parameters::empty(), Some(Type::empty_tuple(db))),
-                            Signature::new(
+                            Signature::new_generic(
+                                Some(GenericContext::from_typevar_instances(db, [element_ty])),
                                 Parameters::new(
                                     db,
                                     [Parameter::positional_only(Some(Name::new_static(
                                         "iterable",
                                     )))
                                     .with_annotated_type(
-                                        KnownClass::Iterable.to_specialized_instance(db, [object]),
+                                        KnownClass::Iterable.to_specialized_instance(
+                                            db,
+                                            [Type::TypeVar(element_ty)],
+                                        ),
                                     )],
                                 ),
-                                Some(Type::homogeneous_tuple(db, object)),
+                                Some(Type::homogeneous_tuple(db, Type::TypeVar(element_ty))),
                             ),
                         ],
                     )

crates/ty_python_semantic/src/types/bound_super.rs

@@ -321,7 +321,7 @@ impl<'db> BoundSuperType<'db> {
             Type::NominalInstance(instance) => SuperOwnerKind::Instance(instance),
 
             Type::ProtocolInstance(protocol) => {
-                if let Some(nominal_instance) = protocol.as_nominal_type() {
+                if let Some(nominal_instance) = protocol.to_nominal_instance() {
                     SuperOwnerKind::Instance(nominal_instance)
                 } else {
                     return Err(BoundSuperError::AbstractOwnerType {

crates/ty_python_semantic/src/types/call/bind.rs

@@ -2818,10 +2818,32 @@ impl<'a, 'db> ArgumentTypeChecker<'a, 'db> {
 
         // Prefer the declared type of generic classes.
         let preferred_type_mappings = return_with_tcx.and_then(|(return_ty, tcx)| {
-            tcx.filter_union(self.db, |ty| ty.class_specialization(self.db).is_some())
-                .class_specialization(self.db)?;
+            let tcx = tcx.filter_union(self.db, |ty| ty.class_specialization(self.db).is_some());
+            let return_ty =
+                return_ty.filter_union(self.db, |ty| ty.class_specialization(self.db).is_some());
+
+            // TODO: We should use the constraint solver here to determine the type mappings for more
+            // complex subtyping relationships, e.g., `type[C[T]]` to `Callable[..., T]`, or unions
+            // containing multiple generic elements.
+            if let Some((class_literal, _)) = return_ty.class_specialization(self.db)
+                && let Some(generic_alias) = class_literal.into_generic_alias()
+            {
+                let specialization = generic_alias.specialization(self.db);
+                for (class_type_var, return_ty) in specialization
+                    .generic_context(self.db)
+                    .variables(self.db)
+                    .zip(specialization.types(self.db))
+                {
+                    if let Some(ty) = tcx.find_type_var_from(
+                        self.db,
+                        class_type_var,
+                        generic_alias.origin(self.db),
+                    ) {
+                        builder.infer(*return_ty, ty).ok()?;
+                    }
+                }
+            }
 
-            builder.infer(return_ty, tcx).ok()?;
             Some(builder.type_mappings().clone())
         });
 

crates/ty_python_semantic/src/types/infer.rs

@@ -381,6 +381,12 @@ impl<'db> TypeContext<'db> {
         }
     }
 
+    pub(crate) fn and_then(self, f: impl FnOnce(Type<'db>) -> Option<Type<'db>>) -> Self {
+        Self {
+            annotation: self.annotation.and_then(f),
+        }
+    }
+
     pub(crate) fn is_typealias(&self) -> bool {
         self.annotation
             .is_some_and(|ty| ty.is_typealias_special_form())