[ty] Allow classes to inherit from type[Any] or type[Unknown] (#18060)

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/mro.md

@@ -177,6 +177,23 @@ if not isinstance(DoesNotExist, type):
     reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
 ```
 
+## Inheritance from `type[Any]` and `type[Unknown]`
+
+Inheritance from `type[Any]` and `type[Unknown]` is also permitted, in keeping with the gradual
+guarantee:
+
+```py
+from typing import Any
+from ty_extensions import Unknown, Intersection
+
+def f(x: type[Any], y: Intersection[Unknown, type[Any]]):
+    class Foo(x): ...
+    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Any, <class 'object'>]
+
+    class Bar(y): ...
+    reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]
+```
+
 ## `__bases__` lists that cause errors at runtime
 
 If the class's `__bases__` cause an exception to be raised at runtime and therefore the class

crates/ty_python_semantic/resources/mdtest/snapshots/mro.md_-_Method_Resolution_Order_tests_-_`__bases__`_lists_with_duplicate_bases.snap

@@ -141,7 +141,7 @@ info[revealed-type]: Revealed type
 ```
 
 ```
-error[duplicate-base]: Duplicate base class `Spam`
+error[duplicate-base]: Duplicate base class `Eggs`
   --> src/mdtest_snippet.py:16:7
    |
 14 |   # error: [duplicate-base] "Duplicate base class `Spam`"
@@ -160,26 +160,25 @@ error[duplicate-base]: Duplicate base class `Spam`
 25 |   # fmt: on
    |
 info: The definition of class `Ham` will raise `TypeError` at runtime
-  --> src/mdtest_snippet.py:17:5
+  --> src/mdtest_snippet.py:18:5
    |
-15 | # error: [duplicate-base] "Duplicate base class `Eggs`"
 16 | class Ham(
 17 |     Spam,
-   |     ---- Class `Spam` first included in bases list here
 18 |     Eggs,
+   |     ---- Class `Eggs` first included in bases list here
 19 |     Bar,
 20 |     Baz,
 21 |     Spam,
-   |     ^^^^ Class `Spam` later repeated here
 22 |     Eggs,
+   |     ^^^^ Class `Eggs` later repeated here
 23 | ): ...
    |
 info: rule `duplicate-base` is enabled by default
 
 ```
 
 ```
-error[duplicate-base]: Duplicate base class `Eggs`
+error[duplicate-base]: Duplicate base class `Spam`
   --> src/mdtest_snippet.py:16:7
    |
 14 |   # error: [duplicate-base] "Duplicate base class `Spam`"
@@ -198,17 +197,18 @@ error[duplicate-base]: Duplicate base class `Eggs`
 25 |   # fmt: on
    |
 info: The definition of class `Ham` will raise `TypeError` at runtime
-  --> src/mdtest_snippet.py:18:5
+  --> src/mdtest_snippet.py:17:5
    |
+15 | # error: [duplicate-base] "Duplicate base class `Eggs`"
 16 | class Ham(
 17 |     Spam,
+   |     ---- Class `Spam` first included in bases list here
 18 |     Eggs,
-   |     ---- Class `Eggs` first included in bases list here
 19 |     Bar,
 20 |     Baz,
 21 |     Spam,
+   |     ^^^^ Class `Spam` later repeated here
 22 |     Eggs,
-   |     ^^^^ Class `Eggs` later repeated here
 23 | ): ...
    |
 info: rule `duplicate-base` is enabled by default

crates/ty_python_semantic/src/types.rs

@@ -543,13 +543,6 @@ impl<'db> Type<'db> {
         Self::Dynamic(DynamicType::Unknown)
     }
 
-    pub(crate) fn into_dynamic(self) -> Option<DynamicType> {
-        match self {
-            Type::Dynamic(dynamic_type) => Some(dynamic_type),
-            _ => None,
-        }
-    }
-
     pub fn object(db: &'db dyn Db) -> Self {
         KnownClass::Object.to_instance(db)
     }

crates/ty_python_semantic/src/types/class_base.rs

@@ -107,16 +107,20 @@ impl<'db> ClassBase<'db> {
             {
                 Self::try_from_type(db, todo_type!("GenericAlias instance"))
             }
+            Type::SubclassOf(subclass_of) => subclass_of
+                .subclass_of()
+                .into_dynamic()
+                .map(ClassBase::Dynamic),
             Type::Intersection(inter) => {
-                let dynamic_element = inter
+                let valid_element = inter
                     .positive(db)
                     .iter()
-                    .find_map(|elem| elem.into_dynamic())?;
+                    .find_map(|elem| ClassBase::try_from_type(db, *elem))?;
 
                 if ty.is_disjoint_from(db, KnownClass::Type.to_instance(db)) {
                     None
                 } else {
-                    Some(ClassBase::Dynamic(dynamic_element))
+                    Some(valid_element)
                 }
             }
             Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
@@ -137,7 +141,6 @@ impl<'db> ClassBase<'db> {
             | Type::LiteralString
             | Type::Tuple(_)
             | Type::ModuleLiteral(_)
-            | Type::SubclassOf(_)
             | Type::TypeVar(_)
             | Type::BoundSuper(_)
             | Type::ProtocolInstance(_)

crates/ty_python_semantic/src/types/subclass_of.rs

@@ -138,6 +138,13 @@ impl<'db> SubclassOfInner<'db> {
         }
     }
 
+    pub(crate) const fn into_dynamic(self) -> Option<DynamicType> {
+        match self {
+            Self::Class(_) => None,
+            Self::Dynamic(dynamic) => Some(dynamic),
+        }
+    }
+
     pub(crate) fn try_from_type(db: &'db dyn Db, ty: Type<'db>) -> Option<Self> {
         match ty {
             Type::Dynamic(dynamic) => Some(Self::Dynamic(dynamic)),