[red-knot] Treat classes as instances of their respective metaclasses in boolean tests (#15105)

## Summary

Follow-up to #15089.

## Test Plan

Markdown tests.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/narrow/truthiness.md

@@ -220,6 +220,57 @@ else:
 reveal_type(y)  # revealed: A & ~AlwaysTruthy | A & ~AlwaysFalsy
 ```
 
+## Truthiness of classes
+
+```py
+class MetaAmbiguous(type):
+    def __bool__(self) -> bool: ...
+
+class MetaFalsy(type):
+    def __bool__(self) -> Literal[False]: ...
+
+class MetaTruthy(type):
+    def __bool__(self) -> Literal[True]: ...
+
+class MetaDeferred(type):
+    def __bool__(self) -> MetaAmbiguous: ...
+
+class AmbiguousClass(metaclass=MetaAmbiguous): ...
+class FalsyClass(metaclass=MetaFalsy): ...
+class TruthyClass(metaclass=MetaTruthy): ...
+class DeferredClass(metaclass=MetaDeferred): ...
+
+def _(
+    a: type[AmbiguousClass],
+    t: type[TruthyClass],
+    f: type[FalsyClass],
+    d: type[DeferredClass],
+    ta: type[TruthyClass | AmbiguousClass],
+    af: type[AmbiguousClass] | type[FalsyClass],
+    flag: bool,
+):
+    reveal_type(ta)  # revealed: type[TruthyClass] | type[AmbiguousClass]
+    if ta:
+        reveal_type(ta)  # revealed: type[TruthyClass] | type[AmbiguousClass] & ~AlwaysFalsy
+
+    reveal_type(af)  # revealed: type[AmbiguousClass] | type[FalsyClass]
+    if af:
+        reveal_type(af)  # revealed: type[AmbiguousClass] & ~AlwaysFalsy
+
+    # TODO: Emit a diagnostic (`d` is not valid in boolean context)
+    if d:
+        # TODO: Should be `Unknown`
+        reveal_type(d)  # revealed: type[DeferredClass] & ~AlwaysFalsy
+
+    tf = TruthyClass if flag else FalsyClass
+    reveal_type(tf)  # revealed: Literal[TruthyClass, FalsyClass]
+
+    if tf:
+        reveal_type(tf)  # revealed: Literal[TruthyClass]
+    else:
+        reveal_type(tf)  # revealed: Literal[FalsyClass]
+```
+
 ## Narrowing in chained boolean expressions
 
 ```py
@@ -253,17 +304,12 @@ class MetaFalsy(type):
     def __bool__(self) -> Literal[False]: ...
 
 class MetaTruthy(type):
-    def __bool__(self) -> Literal[False]: ...
+    def __bool__(self) -> Literal[True]: ...
 
 class FalsyClass(metaclass=MetaFalsy): ...
 class TruthyClass(metaclass=MetaTruthy): ...
 
 def _(x: type[FalsyClass] | type[TruthyClass]):
-    # TODO: Should be `type[TruthyClass] | A`
+    reveal_type(x or A())  # revealed: type[TruthyClass] | A
-    # revealed: type[FalsyClass] & ~AlwaysFalsy | type[TruthyClass] & ~AlwaysFalsy | A
+    reveal_type(x and A())  # revealed: type[FalsyClass] | A
-    reveal_type(x or A())
-
-    # TODO: Should be `type[FalsyClass] | A`
-    # revealed: type[FalsyClass] & ~AlwaysTruthy | type[TruthyClass] & ~AlwaysTruthy | A
-    reveal_type(x and A())
 ```

crates/red_knot_python_semantic/src/types.rs

@@ -1206,14 +1206,6 @@ impl<'db> Type<'db> {
                 Type::SubclassOf(_),
             ) => true,
 
-            (Type::SubclassOf(_), _) | (_, Type::SubclassOf(_)) => {
-                // TODO: Once we have support for final classes, we can determine disjointness in some cases
-                // here. However, note that it might be better to turn `Type::SubclassOf('FinalClass')` into
-                // `Type::ClassLiteral('FinalClass')` during construction, instead of adding special cases for
-                // final classes inside `Type::SubclassOf` everywhere.
-                false
-            }
-
             (Type::AlwaysTruthy, ty) | (ty, Type::AlwaysTruthy) => {
                 // `Truthiness::Ambiguous` may include `AlwaysTrue` as a subset, so it's not guaranteed to be disjoint.
                 // Thus, they are only disjoint if `ty.bool() == AlwaysFalse`.
@@ -1224,6 +1216,14 @@ impl<'db> Type<'db> {
                 matches!(ty.bool(db), Truthiness::AlwaysTrue)
             }
 
+            (Type::SubclassOf(_), _) | (_, Type::SubclassOf(_)) => {
+                // TODO: Once we have support for final classes, we can determine disjointness in some cases
+                // here. However, note that it might be better to turn `Type::SubclassOf('FinalClass')` into
+                // `Type::ClassLiteral('FinalClass')` during construction, instead of adding special cases for
+                // final classes inside `Type::SubclassOf` everywhere.
+                false
+            }
+
             (Type::KnownInstance(left), right) => {
                 left.instance_fallback(db).is_disjoint_from(db, right)
             }
@@ -1678,15 +1678,13 @@ impl<'db> Type<'db> {
             Type::Any | Type::Todo(_) | Type::Never | Type::Unknown => Truthiness::Ambiguous,
             Type::FunctionLiteral(_) => Truthiness::AlwaysTrue,
             Type::ModuleLiteral(_) => Truthiness::AlwaysTrue,
-            Type::ClassLiteral(_) => {
+            Type::ClassLiteral(ClassLiteralType { class }) => {
-                // TODO: lookup `__bool__` and `__len__` methods on the class's metaclass
+                class.metaclass(db).to_instance(db).bool(db)
-                // More info in https://docs.python.org/3/library/stdtypes.html#truth-value-testing
-                Truthiness::Ambiguous
-            }
-            Type::SubclassOf(_) => {
-                // TODO: see above
-                Truthiness::Ambiguous
             }
+            Type::SubclassOf(SubclassOfType { base }) => base
+                .into_class()
+                .map(|class| Type::class_literal(class).bool(db))
+                .unwrap_or(Truthiness::Ambiguous),
             Type::AlwaysTruthy => Truthiness::AlwaysTrue,
             Type::AlwaysFalsy => Truthiness::AlwaysFalse,
             instance_ty @ Type::Instance(InstanceType { class }) => {