[ty] Use constructor parameter types as type context (#21054)

## Summary

Resolves https://github.com/astral-sh/ty/issues/1408.

crates/ty_python_semantic/resources/mdtest/bidirectional.md

@@ -145,3 +145,84 @@ def h[T](x: T, cond: bool) -> T | list[T]:
 def i[T](x: T, cond: bool) -> T | list[T]:
     return x if cond else [x]
 ```
+
+## Type context sources
+
+Type context is sourced from various places, including annotated assignments:
+
+```py
+from typing import Literal
+
+a: list[Literal[1]] = [1]
+```
+
+Function parameter annotations:
+
+```py
+def b(x: list[Literal[1]]): ...
+
+b([1])
+```
+
+Bound method parameter annotations:
+
+```py
+class C:
+    def __init__(self, x: list[Literal[1]]): ...
+    def foo(self, x: list[Literal[1]]): ...
+
+C([1]).foo([1])
+```
+
+Declared variable types:
+
+```py
+d: list[Literal[1]]
+d = [1]
+```
+
+Declared attribute types:
+
+```py
+class E:
+    e: list[Literal[1]]
+
+def _(e: E):
+    # TODO: Implement attribute type context.
+    # error: [invalid-assignment] "Object of type `list[Unknown | int]` is not assignable to attribute `e` of type `list[Literal[1]]`"
+    e.e = [1]
+```
+
+Function return types:
+
+```py
+def f() -> list[Literal[1]]:
+    return [1]
+```
+
+## Class constructor parameters
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+The parameters of both `__init__` and `__new__` are used as type context sources for constructor
+calls:
+
+```py
+def f[T](x: T) -> list[T]:
+    return [x]
+
+class A:
+    def __new__(cls, value: list[int | str]):
+        return super().__new__(cls, value)
+
+    def __init__(self, value: list[int | None]): ...
+
+A(f(1))
+
+# error: [invalid-argument-type] "Argument to function `__new__` is incorrect: Expected `list[int | str]`, found `list[list[Unknown]]`"
+# error: [invalid-argument-type] "Argument to bound method `__init__` is incorrect: Expected `list[int | None]`, found `list[list[Unknown]]`"
+A(f([]))
+```

crates/ty_python_semantic/src/types.rs

@@ -6007,6 +6007,9 @@ impl<'db> Type<'db> {
     /// Given a class literal or non-dynamic `SubclassOf` type, try calling it (creating an instance)
     /// and return the resulting instance type.
     ///
+    /// The `infer_argument_types` closure should be invoked with the signatures of `__new__` and
+    /// `__init__`, such that the argument types can be inferred with the correct type context.
+    ///
     /// Models `type.__call__` behavior.
     /// TODO: model metaclass `__call__`.
     ///
@@ -6017,10 +6020,10 @@ impl<'db> Type<'db> {
     ///
     /// Foo()
     /// ```
-    fn try_call_constructor(
+    fn try_call_constructor<'ast>(
         self,
         db: &'db dyn Db,
-        argument_types: CallArguments<'_, 'db>,
+        infer_argument_types: impl FnOnce(Option<Bindings<'db>>) -> CallArguments<'ast, 'db>,
         tcx: TypeContext<'db>,
     ) -> Result<Type<'db>, ConstructorCallError<'db>> {
         debug_assert!(matches!(
@@ -6076,11 +6079,63 @@ impl<'db> Type<'db> {
         // easy to check if that's the one we found?
         // Note that `__new__` is a static method, so we must inject the `cls` argument.
         let new_method = self_type.lookup_dunder_new(db, ());
+
+        // Construct an instance type that we can use to look up the `__init__` instance method.
+        // This performs the same logic as `Type::to_instance`, except for generic class literals.
+        // TODO: we should use the actual return type of `__new__` to determine the instance type
+        let init_ty = self_type
+            .to_instance(db)
+            .expect("type should be convertible to instance type");
+
+        // Lookup the `__init__` instance method in the MRO.
+        let init_method = init_ty.member_lookup_with_policy(
+            db,
+            "__init__".into(),
+            MemberLookupPolicy::NO_INSTANCE_FALLBACK | MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
+        );
+
+        // Infer the call argument types, using both `__new__` and `__init__` for type-context.
+        let bindings = match (
+            new_method.as_ref().map(|method| &method.place),
+            &init_method.place,
+        ) {
+            (Some(Place::Defined(new_method, ..)), Place::Undefined) => Some(
+                new_method
+                    .bindings(db)
+                    .map(|binding| binding.with_bound_type(self_type)),
+            ),
+
+            (Some(Place::Undefined) | None, Place::Defined(init_method, ..)) => {
+                Some(init_method.bindings(db))
+            }
+
+            (Some(Place::Defined(new_method, ..)), Place::Defined(init_method, ..)) => {
+                let callable = UnionBuilder::new(db)
+                    .add(*new_method)
+                    .add(*init_method)
+                    .build();
+
+                let new_method_bindings = new_method
+                    .bindings(db)
+                    .map(|binding| binding.with_bound_type(self_type));
+
+                Some(Bindings::from_union(
+                    callable,
+                    [new_method_bindings, init_method.bindings(db)],
+                ))
+            }
+
+            _ => None,
+        };
+
+        let argument_types = infer_argument_types(bindings);
+
         let new_call_outcome = new_method.and_then(|new_method| {
             match new_method.place.try_call_dunder_get(db, self_type) {
                 Place::Defined(new_method, _, boundness) => {
                     let result =
                         new_method.try_call(db, argument_types.with_self(Some(self_type)).as_ref());
+
                     if boundness == Definedness::PossiblyUndefined {
                         Some(Err(DunderNewCallError::PossiblyUnbound(result.err())))
                     } else {
@@ -6091,24 +6146,7 @@ impl<'db> Type<'db> {
             }
         });
 
-        // Construct an instance type that we can use to look up the `__init__` instance method.
-        // This performs the same logic as `Type::to_instance`, except for generic class literals.
-        // TODO: we should use the actual return type of `__new__` to determine the instance type
-        let init_ty = self_type
-            .to_instance(db)
-            .expect("type should be convertible to instance type");
-
-        let init_call_outcome = if new_call_outcome.is_none()
-            || !init_ty
-                .member_lookup_with_policy(
-                    db,
-                    "__init__".into(),
-                    MemberLookupPolicy::NO_INSTANCE_FALLBACK
-                        | MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
-                )
-                .place
-                .is_undefined()
-        {
+        let init_call_outcome = if new_call_outcome.is_none() || !init_method.is_undefined() {
             Some(init_ty.try_call_dunder(db, "__init__", argument_types, tcx))
         } else {
             None

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

@@ -100,6 +100,14 @@ impl<'db> Bindings<'db> {
         self.elements.iter()
     }
 
+    pub(crate) fn map(self, f: impl Fn(CallableBinding<'db>) -> CallableBinding<'db>) -> Self {
+        Self {
+            callable_type: self.callable_type,
+            argument_forms: self.argument_forms,
+            elements: self.elements.into_iter().map(f).collect(),
+        }
+    }
+
     /// Match the arguments of a call site against the parameters of a collection of possibly
     /// unioned, possibly overloaded signatures.
     ///

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -6798,9 +6798,6 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     .to_class_type(self.db())
                     .is_none_or(|enum_class| !class.is_subclass_of(self.db(), enum_class))
             {
-                let argument_forms = vec![Some(ParameterForm::Value); call_arguments.len()];
-                self.infer_argument_types(arguments, &mut call_arguments, &argument_forms);
-
                 if matches!(
                     class.known(self.db()),
                     Some(KnownClass::TypeVar | KnownClass::ExtensionsTypeVar)
@@ -6819,8 +6816,21 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     }
                 }
 
+                let db = self.db();
+                let infer_call_arguments = |bindings: Option<Bindings<'db>>| {
+                    if let Some(bindings) = bindings {
+                        let bindings = bindings.match_parameters(self.db(), &call_arguments);
+                        self.infer_all_argument_types(arguments, &mut call_arguments, &bindings);
+                    } else {
+                        let argument_forms = vec![Some(ParameterForm::Value); call_arguments.len()];
+                        self.infer_argument_types(arguments, &mut call_arguments, &argument_forms);
+                    }
+
+                    call_arguments
+                };
+
                 return callable_type
-                    .try_call_constructor(self.db(), call_arguments, tcx)
+                    .try_call_constructor(db, infer_call_arguments, tcx)
                     .unwrap_or_else(|err| {
                         err.report_diagnostic(&self.context, callable_type, call_expression.into());
                         err.return_type()