AST: Fix latent bug in TypeTransformer::transformPackElementType()

Author: slavapestov

include/swift/AST/TypeTransform.h

@@ -1176,6 +1176,12 @@ case TypeKind::Id:
     if (transformedPack.getPointer() == element->getPackType().getPointer())
       return element;
 
+    if (!transformedPack->isParameterPack() &&
+        !transformedPack->is<PackArchetypeType>() &&
+        !transformedPack->isTypeVariableOrMember()) {
+      return transformedPack;
+    }
+
     return PackElementType::get(transformedPack, element->getLevel());
   }
 

lib/AST/ASTContext.cpp

@@ -3879,7 +3879,7 @@ PackElementType::PackElementType(Type packType, unsigned level,
     packType(packType), level(level) {
   assert(packType->isParameterPack() ||
          packType->is<PackArchetypeType>() ||
-         packType->is<TypeVariableType>());
+         packType->isTypeVariableOrMember());
   assert(level > 0);
 }