[ty] support generic aliases in `type[...]`, like `type[C[int]]` (#21552)

Closes https://github.com/astral-sh/ty/issues/1101.

crates/ty_python_semantic/resources/mdtest/diagnostics/same_names.md

@@ -171,7 +171,7 @@ class Config:
 import generic_a
 import generic_b
 
-# TODO should be error: [invalid-assignment] "Object of type `<class 'generic_b.Container[int]'>` is not assignable to `type[generic_a.Container[int]]`"
+# error: [invalid-assignment] "Object of type `<class 'generic_b.Container[int]'>` is not assignable to `type[generic_a.Container[int]]`"
 container: type[generic_a.Container[int]] = generic_b.Container[int]
 ```
 

crates/ty_python_semantic/resources/mdtest/type_of/basic.md

@@ -174,6 +174,39 @@ def _(x: Foo[int], y: Bar[str], z: list[bytes]):
     reveal_type(type(z))  # revealed: type[list[bytes]]
 ```
 
+## Checking generic `type[]` types
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+class C[T]:
+    pass
+
+class D[T]:
+    pass
+
+var: type[C[int]] = C[int]
+var: type[C[int]] = D[int]  # error: [invalid-assignment] "Object of type `<class 'D[int]'>` is not assignable to `type[C[int]]`"
+```
+
+However, generic `Protocol` classes are still TODO:
+
+```py
+from typing import Protocol
+
+class Proto[U](Protocol):
+    def some_method(self): ...
+
+# TODO: should be error: [invalid-assignment]
+var: type[Proto[int]] = C[int]
+
+def _(p: type[Proto[int]]):
+    reveal_type(p)  # revealed: type[@Todo(type[T] for protocols)]
+```
+
 ## `@final` classes
 
 `type[]` types are eagerly converted to class-literal types if a class decorated with `@final` is

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

@@ -243,13 +243,13 @@ static_assert(is_assignable_to(TypeOf[Bar[int]], type[Foo[int]]))
 static_assert(is_assignable_to(TypeOf[Bar[bool]], type[Foo[int]]))
 static_assert(is_assignable_to(TypeOf[Bar], type[Foo[int]]))
 static_assert(is_assignable_to(TypeOf[Bar[Any]], type[Foo[int]]))
+static_assert(is_assignable_to(TypeOf[Bar[Unknown]], type[Foo[int]]))
 static_assert(is_assignable_to(TypeOf[Bar], type[Foo]))
 static_assert(is_assignable_to(TypeOf[Bar[Any]], type[Foo[Any]]))
 static_assert(is_assignable_to(TypeOf[Bar[Any]], type[Foo[int]]))
 
-# TODO: these should pass (all subscripts inside `type[]` type expressions are currently TODO types)
+static_assert(not is_assignable_to(TypeOf[Bar[int]], type[Foo[bool]]))
-static_assert(not is_assignable_to(TypeOf[Bar[int]], type[Foo[bool]]))  # error: [static-assert-error]
+static_assert(not is_assignable_to(TypeOf[Foo[bool]], type[Bar[int]]))
-static_assert(not is_assignable_to(TypeOf[Foo[bool]], type[Bar[int]]))  # error: [static-assert-error]
 ```
 
 ## `type[]` is not assignable to types disjoint from `builtins.type`

crates/ty_python_semantic/src/types.rs

@@ -1566,7 +1566,7 @@ impl<'db> Type<'db> {
                 }
             }
             Type::ClassLiteral(class_literal) => {
-                Some(ClassType::NonGeneric(class_literal).into_callable(db))
+                Some(class_literal.default_specialization(db).into_callable(db))
             }
 
             Type::GenericAlias(alias) => Some(ClassType::Generic(alias).into_callable(db)),
@@ -2406,7 +2406,7 @@ impl<'db> Type<'db> {
                 .subclass_of()
                 .into_class()
                 .map(|subclass_of_class| {
-                    ClassType::NonGeneric(class).has_relation_to_impl(
+                    class.default_specialization(db).has_relation_to_impl(
                         db,
                         subclass_of_class,
                         inferable,
@@ -6707,7 +6707,9 @@ impl<'db> Type<'db> {
                             KnownClass::Float.to_instance(db),
                         ],
                     ),
-                    _ if class.is_typed_dict(db) => Type::typed_dict(*class),
+                    _ if class.is_typed_dict(db) => {
+                        Type::typed_dict(class.default_specialization(db))
+                    }
                     _ => Type::instance(db, class.default_specialization(db)),
                 };
                 Ok(ty)

crates/ty_python_semantic/src/types/class.rs

@@ -362,6 +362,11 @@ impl<'db> VarianceInferable<'db> for GenericAlias<'db> {
     get_size2::GetSize,
 )]
 pub enum ClassType<'db> {
+    // `NonGeneric` is intended to mean that the `ClassLiteral` has no type parameters. There are
+    // places where we currently violate this rule (e.g. so that we print `Foo` instead of
+    // `Foo[Unknown]`), but most callers who need to make a `ClassType` from a `ClassLiteral`
+    // should use `ClassLiteral::default_specialization` instead of assuming
+    // `ClassType::NonGeneric`.
     NonGeneric(ClassLiteral<'db>),
     Generic(GenericAlias<'db>),
 }
@@ -3664,12 +3669,6 @@ impl<'db> From<ClassLiteral<'db>> for Type<'db> {
     }
 }
 
-impl<'db> From<ClassLiteral<'db>> for ClassType<'db> {
-    fn from(class: ClassLiteral<'db>) -> ClassType<'db> {
-        ClassType::NonGeneric(class)
-    }
-}
-
 #[salsa::tracked]
 impl<'db> VarianceInferable<'db> for ClassLiteral<'db> {
     #[salsa::tracked(cycle_initial=crate::types::variance_cycle_initial, heap_size=ruff_memory_usage::heap_size)]

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

@@ -679,11 +679,13 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                 }
                 Type::unknown()
             }
-            ast::Expr::Subscript(ast::ExprSubscript {
+            ast::Expr::Subscript(
-                value,
+                subscript @ ast::ExprSubscript {
-                slice: parameters,
+                    value,
-                ..
+                    slice: parameters,
-            }) => {
+                    ..
+                },
+            ) => {
                 let parameters_ty = match self.infer_expression(value, TypeContext::default()) {
                     Type::SpecialForm(SpecialFormType::Union) => match &**parameters {
                         ast::Expr::Tuple(tuple) => {
@@ -698,6 +700,40 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                         }
                         _ => self.infer_subclass_of_type_expression(parameters),
                     },
+                    value_ty @ Type::ClassLiteral(class_literal) => {
+                        if class_literal.is_protocol(self.db()) {
+                            SubclassOfType::from(
+                                self.db(),
+                                todo_type!("type[T] for protocols").expect_dynamic(),
+                            )
+                        } else {
+                            match class_literal.generic_context(self.db()) {
+                                Some(generic_context) => {
+                                    let db = self.db();
+                                    let specialize = |types: &[Option<Type<'db>>]| {
+                                        SubclassOfType::from(
+                                            db,
+                                            class_literal.apply_specialization(db, |_| {
+                                                generic_context
+                                                    .specialize_partial(db, types.iter().copied())
+                                            }),
+                                        )
+                                    };
+                                    self.infer_explicit_callable_specialization(
+                                        subscript,
+                                        value_ty,
+                                        generic_context,
+                                        specialize,
+                                    )
+                                }
+                                None => {
+                                    // TODO: emit a diagnostic if you try to specialize a non-generic class.
+                                    self.infer_type_expression(parameters);
+                                    todo_type!("specialized non-generic class")
+                                }
+                            }
+                        }
+                    }
                     _ => {
                         self.infer_type_expression(parameters);
                         todo_type!("unsupported nested subscript in type[X]")