Invalid source glyph: 'undefined at required location' when building from .glyphs

[jenskutilek]

Building the attached glyphs file results in this error:

fontc SudoRoman.glyphs
Invalid source glyph 'u': 'undefined at required location Normalized {YTDE: 1.00, wght: 0.00}'

SudoRoman.glyphs.zip

[anthrotype]

hey Jens, thanks for the report!
it seems like there's a composite glyph called "tse-cy" that uses "u" as component; it also has some contours so it needs to be decomposed. However, the "u" base glyph define fewer source locations than the composite glyph "tse-cy" which uses it as a component. "tse-cy" in particular has an extra intermediate layer {400, -64} (a virtual master). In ufo2ft we call these "sparse composite glyphs" whereby locations for a composite and any of its components are not perfectly aligned, either more or less locations in either.
in fontmake (ufo2ft) we support these whereas in fontc we do not yet.
We have another issue open for this here: #954 (comment)
maybe we close this and continue over there

[jenskutilek]

Thank you! It wasn't apparent to me that this was about a composite.

[anthrotype]

Yes, the error message needs improving