Allocate correct size for idxList in TranslateStructBufMatSubscript.

lib/HLSL/HLOperationLower.cpp

@@ -5671,7 +5671,7 @@ void TranslateStructBufMatSubscript(CallInst *CI, Value *handle,
     resultSize = resultType->getVectorNumElements();
   DXASSERT(resultSize <= 16, "up to 4x4 elements in vector or matrix");
   _Analysis_assume_(resultSize <= 16);
-  Value *idxList[16];
+  std::vector<Value *> idxList(resultSize);
 
   switch (subOp) {
   case HLSubscriptOpcode::ColMatSubscript:

tools/clang/test/CodeGenHLSL/shader-compat-suite/matrix_indexing_cs.hlsl

@@ -0,0 +1,14 @@
+// RUN: %dxc -E main -T cs_6_0 %s | FileCheck %s
+
+// Make sure matrix indexing works for struct buf.
+// CHECK: @dx.op.bufferLoad.f32
+// CHECK: @dx.op.bufferStore.f32
+
+StructuredBuffer<matrix<float, 4, 4> > buf1;
+RWBuffer<float4> buf2;
+
+[RootSignature("DescriptorTable(SRV(t0), UAV(u0))")]
+[numthreads(8, 8, 1)]
+void main( uint3 tid : SV_DispatchThreadID) {
+  buf2[tid.x] = buf1[tid.x][tid.y][tid.z];
+}