Register `IDifferentiable` witnesses for the differential associated types of registered types. (#2777)

* Create high-order-forward-diff-struct.slang

* Add a small pass to register differentials for differentials of registered types

* Remove type reg pass and apply logic in differentiable type loading instead

* Update slang-ir-autodiff.cpp

* Fix comments.

---------

Co-authored-by: Yong He <yonghe@outlook.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/source/slang/slang-ir-autodiff.cpp b/source/slang/slang-ir-autodiff.cpp
index 024d31fd8..3257ee102 100644
--- a/source/slang/slang-ir-autodiff.cpp
+++ b/source/slang/slang-ir-autodiff.cpp
@@ -399,6 +399,16 @@ void DifferentiableTypeConformanceContext::setFunc(IRGlobalValueWithCode* func)
             else
             {
                 differentiableWitnessDictionary.Add((IRType*)item->getConcreteType(), item->getWitness());
+
+                // Also register the type's differential type with the same witness.
+                IRBuilder subBuilder(item->getConcreteType());
+                if (!as<IRInterfaceType>(item->getConcreteType()))
+                {
+                    differentiableWitnessDictionary.AddIfNotExists(
+                        (IRType*)_lookupWitness(&subBuilder, item->getWitness(), sharedContext->differentialAssocTypeStructKey), 
+                        item->getWitness());
+                }
+
                 if (auto diffPairType = as<IRDifferentialPairTypeBase>(item->getConcreteType()))
                 {
                     // For differential pair types, register the differential type as well.