HLSL (compute shader, shader model 5_1) conundrum

Question

So I had this HLSL structure that compiled successfully in VS2015 when used:

struct SpatialHashingCellData
{
    uint count;
    uint specks[MAX_SPECKS_PER_CELL];
};

It was used like this:

    gSPCells[cellID].specks[posToWrite]= speckIndex;

But then I switched to VS2017 and I had to make this change in order for it to compile because there was an error: internal error : l-value expected

struct SpatialHashingCellData
{
    uint count;
    struct { uint index; } specks[MAX_SPECKS_PER_CELL];
};

... and changed:

    gSPCells[cellID].specks[posToWrite].index = speckIndex;

I feel kinda stupid for not knowing what is happening here. Hope anyone could clear this out for me, please :)

Best regards, Bojan