[red-knot] Add support for calling type[<dynamic base>]

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/call/subclass_of.md

@@ -0,0 +1,12 @@
+# Call `type[...]`
+
+## Dynamic base
+
+```py
+from typing import Any
+from knot_extensions import Unknown
+
+def _(subclass_of_any: type[Any], subclass_of_unknown: type[Unknown]):
+    reveal_type(subclass_of_any())  # revealed: Any
+    reveal_type(subclass_of_unknown())  # revealed: Unknown
+```

crates/red_knot_python_semantic/src/types.rs

@@ -2678,6 +2678,12 @@ impl<'db> Type<'db> {
                 )))
             }
 
+            Type::SubclassOf(subclass_of) if subclass_of.is_dynamic() => {
+                Ok(CallOutcome::Single(CallBinding::from_return_type(
+                    Type::Dynamic(subclass_of.into_dynamic().expect("checked above")),
+                )))
+            }
+
             instance_ty @ Type::Instance(_) => {
                 instance_ty
                     .try_call_dunder(db, "__call__", arguments)

crates/red_knot_python_semantic/src/types/subclass_of.rs

@@ -1,4 +1,5 @@
 use crate::symbol::SymbolAndQualifiers;
+use crate::types::DynamicType;
 
 use super::{ClassBase, ClassLiteralType, Db, KnownClass, Type};
 
@@ -62,6 +63,13 @@ impl<'db> SubclassOfType<'db> {
         subclass_of.is_dynamic()
     }
 
+    pub fn into_dynamic(self) -> Option<DynamicType> {
+        match self.subclass_of {
+            ClassBase::Dynamic(dynamic) => Some(dynamic),
+            ClassBase::Class(_) => None,
+        }
+    }
+
     pub const fn is_fully_static(self) -> bool {
         !self.is_dynamic()
     }