[merger] Cursive attachment merging fails when all anchors are NULL

[simoncozens]

Carried over from googlefonts/fontmake#1022.

When all anchors in all masters are of the form:

    pos cursive _0 <anchor NULL> <anchor 25 0>;

the merger creates an ExitEntryAnchor like this:

        rec = ot.EntryExitRecord()
        rec.EntryAnchor = ot.Anchor()
        rec.EntryAnchor.Format = 1
        rec.ExitAnchor = ot.Anchor()
        rec.ExitAnchor.Format = 1
        self.EntryExitRecord.append(rec)

This means that deep inside mergeThings, when trying to merge the entry anchors, we hit this code path:

            if allNone(lst):
                if out is not None:
                    raise FoundANone(self, got=lst)

All entry anchors are None, but the target entry anchor is an Anchor so merging fails.

[anthrotype]

is this perhaps a regression introduced with #3209 three weeks ago?

[simoncozens]

Confirmed, it works at 56731a9 but not at b14a29c.

Bad Behdad. ;-)

[anthrotype]

how does one reproduces exactly? could you push a failing test case in varLib/merger_test.py so Behdad can fix it

[simoncozens]

Will do. I could probably fix it myself, TBH; it just needs a custom merger for EntryAnchor/ExitAnchors.