Reading uninitialized value in dynamic loop produces undef with no error/warning

Reading an uninitialized value in a dynamic loop produces no error or warning if the value is also written to in the loop (because undef is permitted in phi nodes, it seems).

source.hlsl:

StructuredBuffer<float> values : register(t0,space0);

void Accumulate(int count, out float result)
{
	// result += values[0];  // <-- This will fail validation and produce an error message
	for (int i = 0; i < count; i++)
		result += values[i];  // <-- This will not
}

float main (int count : IN) : OUT
{
	float result = 0.0;
	Accumulate(count, result);
	return result;
}

dxc.exe -T vs_6_0 source.hlsl

Output:

define void @main() {
  %1 = call %dx.types.Handle @dx.op.createHandle(i32 57, i8 0, i32 0, i32 0, i1 false)  ; CreateHandle(resourceClass,rangeId,index,nonUniformIndex)
  %2 = call i32 @dx.op.loadInput.i32(i32 4, i32 0, i32 0, i8 0, i32 undef)  ; LoadInput(inputSigId,rowIndex,colIndex,gsVertexAxis)
  %3 = icmp sgt i32 %2, 0
  br i1 %3, label %4, label %14

; <label>:4                                       ; preds = %0
  br label %5

; <label>:5                                       ; preds = %5, %4
  %6 = phi i32 [ %11, %5 ], [ 0, %4 ]
  %7 = phi float [ %10, %5 ], [ undef, %4 ]
  %8 = call %dx.types.ResRet.f32 @dx.op.bufferLoad.f32(i32 68, %dx.types.Handle %1, i32 %6, i32 0)  ; BufferLoad(srv,index,wot)
  %9 = extractvalue %dx.types.ResRet.f32 %8, 0
  %10 = fadd fast float %9, %7
  %11 = add nuw nsw i32 %6, 1
  %12 = icmp eq i32 %11, %2
  br i1 %12, label %13, label %5

; <label>:13                                      ; preds = %5
  br label %14

; <label>:14                                      ; preds = %13, %0
  %15 = phi float [ undef, %0 ], [ %10, %13 ]
  call void @dx.op.storeOutput.f32(i32 5, i32 0, i32 0, i8 0, float %15)  ; StoreOutput(outputSigId,rowIndex,colIndex,value)
  ret void
}

Start by reproducing the example in source.hlsl with dxc.exe -T vs_6_0 source.hlsl and compare the dynamic-loop case with the direct access case. Trace the compiler validation path for undef values in the generated phi nodes; done means the loop case produces the appropriate validation error or warning instead of silently emitting undef.