[ty] Correctly instantiate generic class that inherits __init__ from generic base class (#19693)

This is subtle, and the root cause became more apparent with #19604,
since we now have many more cases of superclasses and subclasses using
different typevars. The issue is easiest to see in the following:

```py
class C[T]:
    def __init__(self, t: T) -> None: ...

class D[U](C[T]):
    pass

reveal_type(C(1))  # revealed: C[int]
reveal_type(D(1))  # should be: D[int]
```

When instantiating a generic class, the `__init__` method inherits the
generic context of that class. This lets our call binding machinery
infer a specialization for that context.

Prior to this PR, the instantiation of `C` worked just fine. Its
`__init__` method would inherit the `[T]` generic context, and we would
infer `{T = int}` as the specialization based on the argument
parameters.

It didn't work for `D`. The issue is that the `__init__` method was
inheriting the generic context of the class where `__init__` was defined
(here, `C` and `[T]`). At the call site, we would then infer `{T = int}`
as the specialization — but that wouldn't help us specialize `D[U]`,
since `D` does not have `T` in its generic context!

Instead, the `__init__` method should inherit the generic context of the
class that we are performing the lookup on (here, `D` and `[U]`). That
lets us correctly infer `{U = int}` as the specialization, which we can
successfully apply to `D[U]`.

(Note that `__init__` refers to `C`'s typevars in its signature, but
that's okay; our member lookup logic already applies the `T = U`
specialization when returning a member of `C` while performing a lookup
on `D`, transforming its signature from `(Self, T) -> None` to `(Self,
U) -> None`.)

Closes https://github.com/astral-sh/ty/issues/588

crates/ty_python_semantic/src/types/class.rs

@@ -571,10 +571,20 @@ impl<'db> ClassType<'db> {
     /// Returns the inferred type of the class member named `name`. Only bound members
     /// or those marked as ClassVars are considered.
     ///
+    /// You must provide the `inherited_generic_context` that we should use for the `__new__` or
+    /// `__init__` member. This is inherited from the containing class -­but importantly, from the
+    /// class that the lookup is being performed on, and not the class containing the (possibly
+    /// inherited) member.
+    ///
     /// Returns [`Place::Unbound`] if `name` cannot be found in this class's scope
     /// directly. Use [`ClassType::class_member`] if you require a method that will
     /// traverse through the MRO until it finds the member.
-    pub(super) fn own_class_member(self, db: &'db dyn Db, name: &str) -> PlaceAndQualifiers<'db> {
+    pub(super) fn own_class_member(
+        self,
+        db: &'db dyn Db,
+        inherited_generic_context: Option<GenericContext<'db>>,
+        name: &str,
+    ) -> PlaceAndQualifiers<'db> {
         fn synthesize_getitem_overload_signature<'db>(
             index_annotation: Type<'db>,
             return_annotation: Type<'db>,
@@ -590,7 +600,7 @@ impl<'db> ClassType<'db> {
 
         let fallback_member_lookup = || {
             class_literal
-                .own_class_member(db, specialization, name)
+                .own_class_member(db, inherited_generic_context, specialization, name)
                 .map_type(|ty| ty.apply_optional_specialization(db, specialization))
         };
 
@@ -840,8 +850,11 @@ impl<'db> ClassType<'db> {
                     iterable_parameter,
                 ]);
 
-                let synthesized_dunder =
-                    CallableType::function_like(db, Signature::new(parameters, None));
+                let synthesized_dunder = CallableType::function_like(
+                    db,
+                    Signature::new(parameters, None)
+                        .with_inherited_generic_context(inherited_generic_context),
+                );
 
                 Place::bound(synthesized_dunder).into()
             }
@@ -1668,7 +1681,10 @@ impl<'db> ClassLiteral<'db> {
                     }
 
                     lookup_result = lookup_result.or_else(|lookup_error| {
-                        lookup_error.or_fall_back_to(db, class.own_class_member(db, name))
+                        lookup_error.or_fall_back_to(
+                            db,
+                            class.own_class_member(db, self.generic_context(db), name),
+                        )
                     });
                 }
             }
@@ -1716,6 +1732,7 @@ impl<'db> ClassLiteral<'db> {
     pub(super) fn own_class_member(
         self,
         db: &'db dyn Db,
+        inherited_generic_context: Option<GenericContext<'db>>,
         specialization: Option<Specialization<'db>>,
         name: &str,
     ) -> PlaceAndQualifiers<'db> {
@@ -1744,7 +1761,7 @@ impl<'db> ClassLiteral<'db> {
             // to any method with a `@classmethod` decorator. (`__init__` would remain a special
             // case, since it's an _instance_ method where we don't yet know the generic class's
             // specialization.)
-            match (self.generic_context(db), ty, specialization, name) {
+            match (inherited_generic_context, ty, specialization, name) {
                 (
                     Some(generic_context),
                     Type::FunctionLiteral(function),
@@ -1926,7 +1943,7 @@ impl<'db> ClassLiteral<'db> {
                 KnownClass::NamedTupleFallback
                     .to_class_literal(db)
                     .into_class_literal()?
-                    .own_class_member(db, None, name)
+                    .own_class_member(db, self.generic_context(db), None, name)
                     .place
                     .ignore_possibly_unbound()
             }
@@ -4321,7 +4338,9 @@ enum SlotsKind {
 
 impl SlotsKind {
     fn from(db: &dyn Db, base: ClassLiteral) -> Self {
-        let Place::Type(slots_ty, bound) = base.own_class_member(db, None, "__slots__").place
+        let Place::Type(slots_ty, bound) = base
+            .own_class_member(db, base.generic_context(db), None, "__slots__")
+            .place
         else {
             return Self::NotSpecified;
         };

crates/ty_python_semantic/src/types/signatures.rs

@@ -360,6 +360,14 @@ impl<'db> Signature<'db> {
         Self::new(Parameters::object(db), Some(Type::Never))
     }
 
+    pub(crate) fn with_inherited_generic_context(
+        mut self,
+        inherited_generic_context: Option<GenericContext<'db>>,
+    ) -> Self {
+        self.inherited_generic_context = inherited_generic_context;
+        self
+    }
+
     fn materialize(&self, db: &'db dyn Db, variance: TypeVarVariance) -> Self {
         Self {
             generic_context: self.generic_context,