Fix interface requirement lowering for generic accessors (#6123)

source/slang/slang-lower-to-ir.cpp

@@ -8633,6 +8633,9 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
         UInt operandCount = 0;
         for (auto requirementDecl : decl->members)
         {
+            if (as<GenericDecl>(requirementDecl))
+                requirementDecl = getInner(requirementDecl);
+
             if (as<SubscriptDecl>(requirementDecl) || as<PropertyDecl>(requirementDecl))
             {
                 for (auto accessorDecl : as<ContainerDecl>(requirementDecl)->members)
@@ -8782,6 +8785,13 @@ struct DeclLoweringVisitor : DeclVisitor<DeclLoweringVisitor, LoweredValInfo>
             auto requirementKey = getInterfaceRequirementKey(requirementDecl);
             if (!requirementKey)
             {
+                if (auto genericDecl = as<GenericDecl>(requirementDecl))
+                {
+                    // We need to form a declref into the inner decls in case of a generic
+                    // requirement.
+                    requirementDecl = getInner(genericDecl);
+                }
+
                 if (as<PropertyDecl>(requirementDecl) || as<SubscriptDecl>(requirementDecl))
                 {
                     for (auto member : as<ContainerDecl>(requirementDecl)->members)

tests/autodiff/generic-accessors.slang

@@ -0,0 +1,32 @@
+//TEST:COMPARE_COMPUTE(filecheck-buffer=CHK): -output-using-type
+
+interface ITest
+{
+    __generic<I : IInteger>
+    __subscript(I i) -> float
+    {
+        [BackwardDifferentiable] get;
+    }
+}
+struct Test : ITest
+{
+    __generic<I : IInteger>
+    __subscript(I i) -> float
+    {
+        [BackwardDifferentiable] get { return 5.0f * i.toInt(); }
+    }
+}
+[Differentiable]
+float test(ITest arg)
+{
+    return arg[1];
+}
+//TEST_INPUT:set output = out ubuffer(data=[0 0 0 0], stride=4)
+RWStructuredBuffer<float> output;
+[numthreads(1,1,1)]
+void computeMain()
+{
+    Test t = {};
+    output[0] = test(t);
+    // CHK: 5.0
+}