Fix use of variadic generics with [Differentiable]. (#8736)

There was a bug that causes the compiler failing to treat a `no_diff
TypePack` as a type pack, and thus diagnose an error when resolving the
following call.

The fix is to unwrap any ModifiedType wrappers in `IsTypePack()` check.

2 files changed, 7 insertions, 0 deletions

diff --git a/source/slang/slang-ast-type.cpp b/source/slang/slang-ast-type.cpp
index e3f75cb2f..bfa347df3 100644
--- a/source/slang/slang-ast-type.cpp
+++ b/source/slang/slang-ast-type.cpp
@@ -4,6 +4,7 @@
 #include "slang-ast-builder.h"
 #include "slang-ast-dispatch.h"
 #include "slang-ast-modifier.h"
+#include "slang-check.h"
 #include "slang-syntax.h"
 
 #include <assert.h>
@@ -13,6 +14,7 @@ namespace Slang
 
 bool isAbstractTypePack(Type* type)
 {
+    type = unwrapModifiedType(type);
     if (as<ExpandType>(type))
         return true;
     if (isDeclRefTypeOf<GenericTypePackParamDecl>(type))
@@ -22,6 +24,7 @@ bool isAbstractTypePack(Type* type)
 
 bool isTypePack(Type* type)
 {
+    type = unwrapModifiedType(type);
     if (as<ConcreteTypePack>(type))
         return true;
     return isAbstractTypePack(type);
diff --git a/source/slang/slang-ast-val.h b/source/slang/slang-ast-val.h
index 7b9462f2f..d4edaae08 100644
--- a/source/slang/slang-ast-val.h
+++ b/source/slang/slang-ast-val.h
@@ -1014,6 +1014,10 @@ inline bool isTypeEqualityWitness(Val* witness)
         }
         return true;
     }
+    else if (auto expandWitness = as<ExpandSubtypeWitness>(witness))
+    {
+        return isTypeEqualityWitness(expandWitness->getPatternTypeWitness());
+    }
     return false;
 }