Fix lowering of associated types and synthesis of dispatch functions. (#4568)

* Treat global variables and parameters as non-differentiable when checking derivative data-flow

Global parameters are by-default not differentiable (even if they are of a differentiable type), because our auto-diff passes do not touch anything outside of function bodies.

The solution is to use wrapper objects with differentiable getter/setter methods (and we should provide a few such objects in the stdlib).

Fixes: #3289

This is a potentially breaking change: User code that was previously working with global variables of a differentiable type will now throw an error (previously the gradient would be dropped without warning). The solution is to use `detach()` to keep same behavior as before or rewrite the access using differentiable getter/setter methods.

* Fix issues with lookup witness lowering

* Update slang-ir-lower-witness-lookup.cpp

* Add tests

* Update slang-ir-lower-witness-lookup.cpp

* Cleanup

* Update nested-assoc-types.slang

---------

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

3 files changed, 2 insertions, 8 deletions

diff --git a/source/slang/slang-ir-insts.h b/source/slang/slang-ir-insts.h
index 83b38b3b6..f0fd38061 100644
--- a/source/slang/slang-ir-insts.h
+++ b/source/slang/slang-ir-insts.h
@@ -1292,12 +1292,6 @@ struct IRGetSequentialID : IRInst
     IRInst* getRTTIOperand() { return getOperand(0); }
 };
 
-struct IRLookupWitnessTable : IRInst
-{
-    IRUse sourceType;
-    IRUse interfaceType;
-};
-
 /// Allocates space from local stack.
 ///
 struct IRAlloca : IRInst
diff --git a/source/slang/slang-ir-specialize-dynamic-associatedtype-lookup.cpp b/source/slang/slang-ir-specialize-dynamic-associatedtype-lookup.cpp
index 5a7fd9412..12941469d 100644
--- a/source/slang/slang-ir-specialize-dynamic-associatedtype-lookup.cpp
+++ b/source/slang/slang-ir-specialize-dynamic-associatedtype-lookup.cpp
@@ -158,7 +158,7 @@ struct AssociatedTypeLookupSpecializationContext
         builder.setInsertBefore(inst);
         auto witnessTableArg = inst->getWitnessTable();
         auto callInst = builder.emitCallInst(
-            builder.getWitnessTableIDType(interfaceType), func, witnessTableArg);
+            func->getResultType(), func, witnessTableArg);
         inst->replaceUsesWith(callInst);
         inst->removeAndDeallocate();
     }
diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index 6fa2ce67f..d8d573d63 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -10288,7 +10288,7 @@ static void _addFlattenedTupleArgs(
 
 bool isAbstractWitnessTable(IRInst* inst)
 {
-    if (as<IRThisTypeWitness>(inst))
+    if (as<IRThisTypeWitness>(inst) || as<IRInterfaceRequirementEntry>(inst))
         return true;
     if (auto lookup = as<IRLookupWitnessMethod>(inst))
         return isAbstractWitnessTable(lookup->getWitnessTable());