I think without the . before the name it still counts as a designated initializer, which isn't supported by the C++ 17 standard. I would do this like you did below:

MeshCalcNormalsData data;
data.mpolys = mpolys;
data.mloop = mloop;
...

Please check for other places like that in these changes. I see one a few lines above with the same struct.

This one stumped me for a bit, my initial reaction was this shouldn't even build, but it does...

this is due to name of the fields inside the struct and the variables we put in having identical names, and the way side effects work, this essentially evaluates to:

MeshCalcNormalsData data = {
  mpolys,
  mloop,
...

So as long as the order in the struct matches the order in the initializer, this will work, but

1. it doesn't do what you think it does
2. if the order of the struct ever changes, it would break. ... likely silently

so follow @Hans Goudey (HooglyBoogly) 's guidance here.