use nullptr' for IRStructKey with IRDerivativeMemberDecoration` (#4623)

Author: ArielG-NV

source/slang/slang-emit.cpp

@@ -292,6 +292,7 @@ void calcRequiredLoweringPassSet(RequiredLoweringPassSet& result, CodeGenContext
     case kIROp_BackwardDifferentiate:
     case kIROp_ForwardDifferentiate:
     case kIROp_MakeDifferentialPairUserCode:
+    case kIROp_DerivativeMemberDecoration:
         result.autodiff = true;
         break;
     case kIROp_VerticesType:

tests/compute/struct-autodiff-default-init.slang

@@ -0,0 +1,25 @@
+// struct-default-init.slang
+//TEST:SIMPLE(filecheck=HLSL): -target hlsl -entry computeMain -stage compute
+//TEST:SIMPLE(filecheck=GLSL): -target glsl -entry computeMain -stage compute
+//TEST:SIMPLE(filecheck=METAL): -target metal -entry computeMain -stage compute
+//TEST:SIMPLE(filecheck=CPP): -target cpp -entry computeMain -stage compute
+//TEST:SIMPLE(filecheck=SPIRV): -target spirv -entry computeMain -stage compute
+
+// HLSL: computeMain
+// GLSL: main
+// METAL: computeMain
+// CPP: computeMain
+// SPIRV: OpEntryPoint
+
+struct PowActivationEx : IDifferentiable
+{
+	float power;
+}
+RWStructuredBuffer<float> return_value;
+
+[numthreads(256, 1, 1)]
+void computeMain()
+{
+    PowActivationEx args;
+	return_value[0] = args.power;
+}