[ty] Add subtyping between Callable types and class literals with __init__ (#17638)

## Summary

Allow classes with `__init__` to be subtypes of `Callable`

Fixes https://github.com/astral-sh/ty/issues/358

## Test Plan

Update is_subtype_of.md

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

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

@@ -608,11 +608,49 @@ c: Callable[[Any], str] = A().g
 ```py
 from typing import Any, Callable
 
+c: Callable[[object], type] = type
 c: Callable[[str], Any] = str
 c: Callable[[str], Any] = int
 
 # error: [invalid-assignment]
 c: Callable[[str], Any] = object
+
+class A:
+    def __init__(self, x: int) -> None: ...
+
+a: Callable[[int], A] = A
+
+class C:
+    def __new__(cls, *args, **kwargs) -> "C":
+        return super().__new__(cls)
+
+    def __init__(self, x: int) -> None: ...
+
+c: Callable[[int], C] = C
+```
+
+### Generic class literal types
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from typing import Callable
+
+class B[T]:
+    def __init__(self, x: T) -> None: ...
+
+b: Callable[[int], B[int]] = B[int]
+
+class C[T]:
+    def __new__(cls, *args, **kwargs) -> "C[T]":
+        return super().__new__(cls)
+
+    def __init__(self, x: T) -> None: ...
+
+c: Callable[[int], C[int]] = C[int]
 ```
 
 ### Overloads

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -1219,7 +1219,7 @@ static_assert(is_subtype_of(TypeOf[C], Callable[[], str]))
 #### Classes with `__new__`
 
 ```py
-from typing import Callable
+from typing import Callable, overload
 from ty_extensions import TypeOf, static_assert, is_subtype_of
 
 class A:
@@ -1244,6 +1244,20 @@ static_assert(is_subtype_of(TypeOf[E], Callable[[], C]))
 static_assert(is_subtype_of(TypeOf[E], Callable[[], B]))
 static_assert(not is_subtype_of(TypeOf[D], Callable[[], C]))
 static_assert(is_subtype_of(TypeOf[D], Callable[[], B]))
+
+class F:
+    @overload
+    def __new__(cls) -> int: ...
+    @overload
+    def __new__(cls, x: int) -> "F": ...
+    def __new__(cls, x: int | None = None) -> "int | F":
+        return 1 if x is None else object.__new__(cls)
+
+    def __init__(self, y: str) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[F], Callable[[int], F]))
+static_assert(is_subtype_of(TypeOf[F], Callable[[], int]))
+static_assert(not is_subtype_of(TypeOf[F], Callable[[str], F]))
 ```
 
 #### Classes with `__call__` and `__new__`
@@ -1266,6 +1280,123 @@ static_assert(is_subtype_of(TypeOf[F], Callable[[], int]))
 static_assert(not is_subtype_of(TypeOf[F], Callable[[], str]))
 ```
 
+#### Classes with `__init__`
+
+```py
+from typing import Callable, overload
+from ty_extensions import TypeOf, static_assert, is_subtype_of
+
+class A:
+    def __init__(self, a: int) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[A], Callable[[int], A]))
+static_assert(not is_subtype_of(TypeOf[A], Callable[[], A]))
+
+class B:
+    @overload
+    def __init__(self, a: int) -> None: ...
+    @overload
+    def __init__(self) -> None: ...
+    def __init__(self, a: int | None = None) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[B], Callable[[int], B]))
+static_assert(is_subtype_of(TypeOf[B], Callable[[], B]))
+
+class C: ...
+
+# TODO: This assertion should be true once we understand `Self`
+# error: [static-assert-error] "Static assertion error: argument evaluates to `False`"
+static_assert(is_subtype_of(TypeOf[C], Callable[[], C]))
+
+class D[T]:
+    def __init__(self, x: T) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[D[int]], Callable[[int], D[int]]))
+static_assert(not is_subtype_of(TypeOf[D[int]], Callable[[str], D[int]]))
+```
+
+#### Classes with `__init__` and `__new__`
+
+```py
+from typing import Callable, overload, Self
+from ty_extensions import TypeOf, static_assert, is_subtype_of
+
+class A:
+    def __new__(cls, a: int) -> Self:
+        return super().__new__(cls)
+
+    def __init__(self, a: int) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[A], Callable[[int], A]))
+static_assert(not is_subtype_of(TypeOf[A], Callable[[], A]))
+
+class B:
+    def __new__(cls, a: int) -> int:
+        return super().__new__(cls)
+
+    def __init__(self, a: str) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[B], Callable[[int], int]))
+static_assert(not is_subtype_of(TypeOf[B], Callable[[str], B]))
+
+class C:
+    def __new__(cls, *args, **kwargs) -> "C":
+        return super().__new__(cls)
+
+    def __init__(self, x: int) -> None: ...
+
+# Not subtype because __new__ signature is not fully static
+static_assert(not is_subtype_of(TypeOf[C], Callable[[int], C]))
+static_assert(not is_subtype_of(TypeOf[C], Callable[[], C]))
+
+class D: ...
+
+class E:
+    @overload
+    def __new__(cls) -> int: ...
+    @overload
+    def __new__(cls, x: int) -> D: ...
+    def __new__(cls, x: int | None = None) -> int | D:
+        return D()
+
+    def __init__(self, y: str) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[E], Callable[[int], D]))
+static_assert(is_subtype_of(TypeOf[E], Callable[[], int]))
+
+class F[T]:
+    def __new__(cls, x: T) -> "F[T]":
+        return super().__new__(cls)
+
+    def __init__(self, x: T) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[F[int]], Callable[[int], F[int]]))
+static_assert(not is_subtype_of(TypeOf[F[int]], Callable[[str], F[int]]))
+```
+
+#### Classes with `__call__`, `__new__` and `__init__`
+
+If `__call__`, `__new__` and `__init__` are all present, `__call__` takes precedence.
+
+```py
+from typing import Callable
+from ty_extensions import TypeOf, static_assert, is_subtype_of
+
+class MetaWithIntReturn(type):
+    def __call__(cls) -> int:
+        return super().__call__()
+
+class F(metaclass=MetaWithIntReturn):
+    def __new__(cls) -> str:
+        return super().__new__(cls)
+
+    def __init__(self, x: int) -> None: ...
+
+static_assert(is_subtype_of(TypeOf[F], Callable[[], int]))
+static_assert(not is_subtype_of(TypeOf[F], Callable[[], str]))
+static_assert(not is_subtype_of(TypeOf[F], Callable[[int], F]))
+```
+
 ### Bound methods
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -1351,12 +1351,15 @@ impl<'db> Type<'db> {
             }
 
             (Type::ClassLiteral(class_literal), Type::Callable(_)) => {
-                if let Some(callable) = class_literal.into_callable(db) {
-                    return callable.is_subtype_of(db, target);
-                }
-                false
+                ClassType::NonGeneric(class_literal)
+                    .into_callable(db)
+                    .is_subtype_of(db, target)
             }
 
+            (Type::GenericAlias(alias), Type::Callable(_)) => ClassType::Generic(alias)
+                .into_callable(db)
+                .is_subtype_of(db, target),
+
             // `Literal[str]` is a subtype of `type` because the `str` class object is an instance of its metaclass `type`.
             // `Literal[abc.ABC]` is a subtype of `abc.ABCMeta` because the `abc.ABC` class object
             // is an instance of its metaclass `abc.ABCMeta`.
@@ -1656,12 +1659,15 @@ impl<'db> Type<'db> {
             }
 
             (Type::ClassLiteral(class_literal), Type::Callable(_)) => {
-                if let Some(callable) = class_literal.into_callable(db) {
-                    return callable.is_assignable_to(db, target);
-                }
-                false
+                ClassType::NonGeneric(class_literal)
+                    .into_callable(db)
+                    .is_assignable_to(db, target)
             }
 
+            (Type::GenericAlias(alias), Type::Callable(_)) => ClassType::Generic(alias)
+                .into_callable(db)
+                .is_assignable_to(db, target),
+
             (Type::FunctionLiteral(self_function_literal), Type::Callable(_)) => {
                 self_function_literal
                     .into_callable_type(db)

crates/ty_python_semantic/src/types/class.rs

@@ -9,7 +9,7 @@ use super::{
 use crate::semantic_index::DeclarationWithConstraint;
 use crate::semantic_index::definition::Definition;
 use crate::types::generics::{GenericContext, Specialization};
-use crate::types::signatures::{Parameter, Parameters, Signature};
+use crate::types::signatures::{CallableSignature, Parameter, Parameters, Signature};
 use crate::types::{
     CallableType, DataclassParams, DataclassTransformerParams, KnownInstanceType, TypeMapping,
     TypeVarInstance,
@@ -493,6 +493,142 @@ impl<'db> ClassType<'db> {
             .own_instance_member(db, name)
             .map_type(|ty| ty.apply_optional_specialization(db, specialization))
     }
+
+    /// Return a callable type (or union of callable types) that represents the callable
+    /// constructor signature of this class.
+    pub(super) fn into_callable(self, db: &'db dyn Db) -> Type<'db> {
+        let self_ty = Type::from(self);
+        let metaclass_dunder_call_function_symbol = self_ty
+            .member_lookup_with_policy(
+                db,
+                "__call__".into(),
+                MemberLookupPolicy::NO_INSTANCE_FALLBACK
+                    | MemberLookupPolicy::META_CLASS_NO_TYPE_FALLBACK,
+            )
+            .symbol;
+
+        if let Symbol::Type(Type::BoundMethod(metaclass_dunder_call_function), _) =
+            metaclass_dunder_call_function_symbol
+        {
+            // TODO: this intentionally diverges from step 1 in
+            // https://typing.python.org/en/latest/spec/constructors.html#converting-a-constructor-to-callable
+            // by always respecting the signature of the metaclass `__call__`, rather than
+            // using a heuristic which makes unwarranted assumptions to sometimes ignore it.
+            return metaclass_dunder_call_function.into_callable_type(db);
+        }
+
+        let dunder_new_function_symbol = self_ty
+            .member_lookup_with_policy(
+                db,
+                "__new__".into(),
+                MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
+            )
+            .symbol;
+
+        let dunder_new_function =
+            if let Symbol::Type(Type::FunctionLiteral(dunder_new_function), _) =
+                dunder_new_function_symbol
+            {
+                // Step 3: If the return type of the `__new__` evaluates to a type that is not a subclass of this class,
+                // then we should ignore the `__init__` and just return the `__new__` method.
+                let returns_non_subclass =
+                    dunder_new_function
+                        .signature(db)
+                        .overloads
+                        .iter()
+                        .any(|signature| {
+                            signature.return_ty.is_some_and(|return_ty| {
+                                !return_ty.is_assignable_to(
+                                    db,
+                                    self_ty
+                                        .to_instance(db)
+                                        .expect("ClassType should be instantiable"),
+                                )
+                            })
+                        });
+
+                let dunder_new_bound_method =
+                    dunder_new_function.into_bound_method_type(db, self_ty);
+
+                if returns_non_subclass {
+                    return dunder_new_bound_method;
+                }
+                Some(dunder_new_bound_method)
+            } else {
+                None
+            };
+
+        let dunder_init_function_symbol = self_ty
+            .member_lookup_with_policy(
+                db,
+                "__init__".into(),
+                MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK
+                    | MemberLookupPolicy::META_CLASS_NO_TYPE_FALLBACK,
+            )
+            .symbol;
+
+        let correct_return_type = self_ty.to_instance(db).unwrap_or_else(Type::unknown);
+
+        // If the class defines an `__init__` method, then we synthesize a callable type with the
+        // same parameters as the `__init__` method after it is bound, and with the return type of
+        // the concrete type of `Self`.
+        let synthesized_dunder_init_callable =
+            if let Symbol::Type(Type::FunctionLiteral(dunder_init_function), _) =
+                dunder_init_function_symbol
+            {
+                let synthesized_signature = |signature: Signature<'db>| {
+                    Signature::new(signature.parameters().clone(), Some(correct_return_type))
+                        .bind_self()
+                };
+
+                let synthesized_dunder_init_signature = CallableSignature::from_overloads(
+                    dunder_init_function
+                        .signature(db)
+                        .overloads
+                        .iter()
+                        .cloned()
+                        .map(synthesized_signature),
+                );
+                Some(Type::Callable(CallableType::new(
+                    db,
+                    synthesized_dunder_init_signature,
+                    true,
+                )))
+            } else {
+                None
+            };
+
+        match (dunder_new_function, synthesized_dunder_init_callable) {
+            (Some(dunder_new_function), Some(synthesized_dunder_init_callable)) => {
+                UnionType::from_elements(
+                    db,
+                    vec![dunder_new_function, synthesized_dunder_init_callable],
+                )
+            }
+            (Some(constructor), None) | (None, Some(constructor)) => constructor,
+            (None, None) => {
+                // If no `__new__` or `__init__` method is found, then we fall back to looking for
+                // an `object.__new__` method.
+                let new_function_symbol = self_ty
+                    .member_lookup_with_policy(
+                        db,
+                        "__new__".into(),
+                        MemberLookupPolicy::META_CLASS_NO_TYPE_FALLBACK,
+                    )
+                    .symbol;
+
+                if let Symbol::Type(Type::FunctionLiteral(new_function), _) = new_function_symbol {
+                    new_function.into_bound_method_type(db, self_ty)
+                } else {
+                    // Fallback if no `object.__new__` is found.
+                    CallableType::single(
+                        db,
+                        Signature::new(Parameters::empty(), Some(correct_return_type)),
+                    )
+                }
+            }
+        }
+    }
 }
 
 impl<'db> From<GenericAlias<'db>> for ClassType<'db> {
@@ -991,40 +1127,6 @@ impl<'db> ClassLiteral<'db> {
         ))
     }
 
-    pub(super) fn into_callable(self, db: &'db dyn Db) -> Option<Type<'db>> {
-        let self_ty = Type::from(self);
-        let metaclass_call_function_symbol = self_ty
-            .member_lookup_with_policy(
-                db,
-                "__call__".into(),
-                MemberLookupPolicy::NO_INSTANCE_FALLBACK
-                    | MemberLookupPolicy::META_CLASS_NO_TYPE_FALLBACK,
-            )
-            .symbol;
-
-        if let Symbol::Type(Type::BoundMethod(metaclass_call_function), _) =
-            metaclass_call_function_symbol
-        {
-            // TODO: this intentionally diverges from step 1 in
-            // https://typing.python.org/en/latest/spec/constructors.html#converting-a-constructor-to-callable
-            // by always respecting the signature of the metaclass `__call__`, rather than
-            // using a heuristic which makes unwarranted assumptions to sometimes ignore it.
-            return Some(metaclass_call_function.into_callable_type(db));
-        }
-
-        let dunder_new_method = self_ty
-            .find_name_in_mro(db, "__new__")
-            .expect("find_name_in_mro always succeeds for class literals")
-            .symbol
-            .try_call_dunder_get(db, self_ty);
-
-        if let Symbol::Type(Type::FunctionLiteral(dunder_new_method), _) = dunder_new_method {
-            return Some(dunder_new_method.into_bound_method_type(db, self.into()));
-        }
-        // TODO handle `__init__` also
-        None
-    }
-
     /// Returns the class member of this class named `name`.
     ///
     /// The member resolves to a member on the class itself or any of its proper superclasses.

crates/ty_python_semantic/src/types/known_instance.rs

@@ -88,7 +88,8 @@ pub enum KnownInstanceType<'db> {
     /// The symbol `typing.Callable`
     /// (which can also be found as `typing_extensions.Callable` or as `collections.abc.Callable`)
     Callable,
-    /// The symbol `typing.Self` (which can also be found as `typing_extensions.Self`)
+    /// The symbol `typing.Self` (which can also be found as `typing_extensions.Self` or
+    /// `_typeshed.Self`)
     TypingSelf,
 
     // Various special forms, special aliases and type qualifiers that we don't yet understand
@@ -307,7 +308,6 @@ impl<'db> KnownInstanceType<'db> {
             | Self::Literal
             | Self::LiteralString
             | Self::Never
-            | Self::TypingSelf
             | Self::Final
             | Self::Concatenate
             | Self::Unpack
@@ -322,6 +322,12 @@ impl<'db> KnownInstanceType<'db> {
             | Self::TypeVar(_) => {
                 matches!(module, KnownModule::Typing | KnownModule::TypingExtensions)
             }
+            Self::TypingSelf => {
+                matches!(
+                    module,
+                    KnownModule::Typing | KnownModule::TypingExtensions | KnownModule::Typeshed
+                )
+            }
             Self::Unknown
             | Self::AlwaysTruthy
             | Self::AlwaysFalsy