[ty] Allow a class to inherit from an intersection if the intersection contains a dynamic type and the intersection is not disjoint from type (#18055)

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/mro.md

@@ -154,6 +154,29 @@ reveal_type(E.__mro__)  # revealed: tuple[<class 'E'>, <class 'B'>, <class 'C'>,
 reveal_type(F.__mro__)
 ```
 
+## Inheritance with intersections that include `Unknown`
+
+An intersection that includes `Unknown` or `Any` is permitted as long as the intersection is not
+disjoint from `type`.
+
+```py
+from does_not_exist import DoesNotExist  # error: [unresolved-import]
+
+reveal_type(DoesNotExist)  # revealed: Unknown
+
+if hasattr(DoesNotExist, "__mro__"):
+    reveal_type(DoesNotExist)  # revealed: Unknown & <Protocol with members '__mro__'>
+
+    class Foo(DoesNotExist): ...  # no error!
+    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
+
+if not isinstance(DoesNotExist, type):
+    reveal_type(DoesNotExist)  # revealed: Unknown & ~type
+
+    class Foo(DoesNotExist): ...  # error: [invalid-base]
+    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, 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/src/types.rs

@@ -543,6 +543,13 @@ 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

@@ -107,8 +107,19 @@ impl<'db> ClassBase<'db> {
             {
                 Self::try_from_type(db, todo_type!("GenericAlias instance"))
             }
+            Type::Intersection(inter) => {
+                let dynamic_element = inter
+                    .positive(db)
+                    .iter()
+                    .find_map(|elem| elem.into_dynamic())?;
+
+                if ty.is_disjoint_from(db, KnownClass::Type.to_instance(db)) {
+                    None
+                } else {
+                    Some(ClassBase::Dynamic(dynamic_element))
+                }
+            }
             Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
-            Type::Intersection(_) => None, // TODO -- probably incorrect?
             Type::NominalInstance(_) => None, // TODO -- handle `__mro_entries__`?
             Type::PropertyInstance(_) => None,
             Type::Never