compile.el puts note in the same capture group as info — group 7, level 0 —
while warning is group 6, level 1, and compilation-skip-threshold defaults to
1. So next-error walks the errors with no configuration, which is the claim
M-x compile rests on and it holds; it steps over the notes until the
threshold is 0. They are still parsed, coloured and clickable.
Labelling notes warning: would make them navigable at the default and is
refused. A note is not a warning, and a compile whose only complaint is an
error would start reporting warnings that are not warnings.
The macro expansion field gets the test it was missing, through a real
expansion rather than a unit test on either half: the tag is put on by Macro
and defaulted into the diagnostic by Loc, and either half alone would pass
with the other broken. clamp misused expands into a call to a name that does
not exist, so the checker refuses something the author never wrote, which is
the case the field is for.