Initial support for differentiating existential types (#3111)

* Merge

* WIP: Complete auto-diff logic for existential types

* Revert "Add compiler option for generating representative hash"

This reverts commit 13b09ef4621e73844c96d64d9c111a8ed0d45aae.

* More fixes for fwd-mode AD on existential types

* Add anyValueSize inference pass

* Fix checking of `Differential.Differential==Differential`

* In-progress: infer any-value-size for existential types

* Existentials now work in forward-mode

* Overhaul handling of existential AD types. Fwd-mode works, reverse-mode requires front-end changes

* Reverse-mode now works on existentials

* Cleanup

* Remove diff rules for create existential object for now

* Revert treat-as-differentiable changes

* Fixes

* More fixes

* Cleanup

* more cleanup

* signed/unsigned

* Revert "Cleanup"

This reverts commit e4f7d71f07bb207736f90708961eeecd09a1b652.

* Cleanup (again)

* Remove public/export/keep-alive on null differential after AD pass

* Minor fix

* Update dictionary accessors

* Keep export decoration

* More fixes + Support for `kIROp_PackAnyValue`

* Merge upstream

* Update expected-failure.txt

1 files changed, 20 insertions, 2 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index cb3db9e39..f25821dac 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -1656,8 +1656,7 @@ namespace Slang
             RequirementWitness witnessValue;
             auto requirementDecl = m_astBuilder->getSharedASTBuilder()->findBuiltinRequirementDecl(BuiltinRequirementKind::DifferentialType);
             if (!inheritanceDecl->witnessTable->getRequirementDictionary().tryGetValue(requirementDecl, witnessValue))
-                return;
-
+                return;            
             // A type used as differential type must have itself as its own differential type.
             if (witnessValue.getFlavor() != RequirementWitness::Flavor::val)
                 return;
@@ -5781,6 +5780,16 @@ namespace Slang
                 interfaceDecl->members.add(reqDecl);
                 reqDecl->parentDecl = interfaceDecl;
 
+                if (!decl->hasModifier<NoDiffThisAttribute>())
+                {
+                    // Build decl-ref-type from interface.
+                    auto interfaceType = DeclRefType::create(getASTBuilder(), makeDeclRef(interfaceDecl));
+
+                    // If the interface is differentiable, make the this type a pair.
+                    if (tryGetDifferentialType(getASTBuilder(), interfaceType))
+                        reqDecl->diffThisType = getDifferentialPairType(interfaceType);
+                }
+
                 auto reqRef = m_astBuilder->create<DerivativeRequirementReferenceDecl>();
                 reqRef->referencedDecl = reqDecl;
                 reqRef->parentDecl = decl;
@@ -5800,6 +5809,15 @@ namespace Slang
                     setFuncTypeIntoRequirementDecl(reqDecl, diffFuncType);
                     interfaceDecl->members.add(reqDecl);
                     reqDecl->parentDecl = interfaceDecl;
+                    if (!decl->hasModifier<NoDiffThisAttribute>())
+                    {
+                        // Build decl-ref-type from interface.
+                        auto interfaceType = DeclRefType::create(getASTBuilder(), makeDeclRef(interfaceDecl));
+
+                        // If the interface is differentiable, make the this type a pair.
+                        if (tryGetDifferentialType(getASTBuilder(), interfaceType))
+                            reqDecl->diffThisType = getDifferentialPairType(interfaceType);
+                    }
 
                     auto reqRef = m_astBuilder->create<DerivativeRequirementReferenceDecl>();
                     reqRef->referencedDecl = reqDecl;