[Sema] Don't wrap class types in existential types.

This could happen previously when computing the existential type
for a given archetype. In cases where an archetype is only constrained
to a class, return the class type directly rather than wrapping it
in an existential type.

Author: hborla

lib/AST/ASTContext.cpp

@@ -4281,6 +4281,8 @@ Type ExistentialType::get(Type constraint) {
   if (constraint->is<ExistentialMetatypeType>())
     return constraint;
 
+  assert(constraint->isConstraintType());
+
   auto properties = constraint->getRecursiveProperties();
   auto arena = getArena(properties);
 

lib/AST/Type.cpp

@@ -3362,6 +3362,13 @@ Type ArchetypeType::getExistentialType() const {
   auto constraint = ProtocolCompositionType::get(
      ctx, constraintTypes, requiresClass());
 
+  // If the archetype is only constrained to a class type,
+  // return the class type directly.
+  if (!constraint->isConstraintType()) {
+    assert(constraint->getClassOrBoundGenericClass());
+    return constraint;
+  }
+
   return ExistentialType::get(constraint);
 }
 

validation-test/compiler_crashers_2_fixed/rdar88296943.sil

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+import Swift
+
+class C {}
+sil_vtable C {}
+
+sil @repo: $@convention(c) @pseudogeneric <S, I where S : AnyObject, S : Hashable, I : C> (@inout_aliasable @block_storage @callee_guaranteed (@guaranteed S, Int, UnsafeMutablePointer<Bool>) -> (), S, Int, UnsafeMutablePointer<Bool>) -> () {
+bb0(%0 : $*@block_storage @callee_guaranteed (@guaranteed S, Int, UnsafeMutablePointer<Bool>) -> (), %1 : $S, %2 : $Int, %3 : $UnsafeMutablePointer<Bool>):
+  %9 = tuple ()                                   // user: %10
+  return %9 : $()                                 // id: %10
+}