__is_transparent_v used to accept an additional _Key template argument
whose sole purpose was to make the instantiation as a whole dependent.
It turns out that creates confusion around whether that trait takes into
account the key type (it does not). Instead, we can use our traditional
approach for making template params artificially dependent, which allows
removing the confusing parameter.
For disclaimer, I authored this patch with Claude code just to see if I
could get it to do the right thing. It works, but you have to steer it
right.
Fixes#186417
fc569dafd7