Fix lowering of associated types in generic interfaces (#6600)

* Fix lowering of associated types in generic interfaces.

* Update diff-assoctype-generic-interface.slang

* Fix-up lowering of differentiable witnesses for implicit ops

* Update slang-ir-autodiff-transcriber-base.cpp

* Fix issue with differentiating type-packs

1 files changed, 4 insertions, 32 deletions

diff --git a/source/slang/slang-ir-autodiff-transcriber-base.cpp b/source/slang/slang-ir-autodiff-transcriber-base.cpp
index 8356e5f81..d67d75997 100644
--- a/source/slang/slang-ir-autodiff-transcriber-base.cpp
+++ b/source/slang/slang-ir-autodiff-transcriber-base.cpp
@@ -720,9 +720,7 @@ IRInst* AutoDiffTranscriberBase::getDifferentialZeroOfType(IRBuilder* builder, I
 
     if (auto diffType = differentiateType(builder, originalType))
     {
-        IRInst* diffWitnessTable = nullptr;
-        IRType* diffOuterType = nullptr;
-        if (isExistentialType(diffType))
+        if (isExistentialType(diffType) && !as<IRLookupWitnessMethod>(diffType))
         {
             // Emit null differential & pack it into an IDifferentiable existential.
 
@@ -789,25 +787,8 @@ IRInst* AutoDiffTranscriberBase::getDifferentialZeroOfType(IRBuilder* builder, I
             return result;
         }
 
-        // Since primalType has a corresponding differential type, we can lookup the
-        // definition for zero().
-        IRInst* zeroMethod = nullptr;
-        if (auto lookupInterface = as<IRLookupWitnessMethod>(diffType))
-        {
-            // if the differential type itself comes from a witness lookup, we can just lookup the
-            // zero method from the same witness table.
-            auto wt = lookupInterface->getWitnessTable();
-            zeroMethod = builder->emitLookupInterfaceMethodInst(
-                builder->getFuncType(List<IRType*>(), diffType),
-                wt,
-                autoDiffSharedContext->zeroMethodStructKey);
-            builder->markInstAsPrimal(zeroMethod);
-        }
-        else
-        {
-            zeroMethod =
-                differentiableTypeConformanceContext.getZeroMethodForType(builder, originalType);
-        }
+        auto zeroMethod =
+            differentiableTypeConformanceContext.getZeroMethodForType(builder, originalType);
         SLANG_RELEASE_ASSERT(zeroMethod);
 
         auto emptyArgList = List<IRInst*>();
@@ -815,16 +796,7 @@ IRInst* AutoDiffTranscriberBase::getDifferentialZeroOfType(IRBuilder* builder, I
         auto callInst = builder->emitCallInst((IRType*)diffType, zeroMethod, emptyArgList);
         builder->markInstAsDifferential(callInst, primalType);
 
-        if (diffOuterType && isExistentialType(diffOuterType))
-        {
-            // Need to wrap the result back into an existential.
-            auto existentialZero =
-                builder->emitMakeExistential(diffOuterType, callInst, diffWitnessTable);
-            builder->markInstAsDifferential(existentialZero, primalType);
-            return existentialZero;
-        }
-        else
-            return callInst;
+        return callInst;
     }
     else
     {