Somewhat vague error message when a corner component blocks variable font build `Generating fonts from Designspace failed: 'origin'`

[arrowtype]

I’ve just experienced some confusion in a build, and I wanted to document it in case others come across this.

What happened

I was trying to build from a relatively simple Glyphs document to a variable font.

Here’s the log:

▶ fontmake -o variable -g Familyname/sources/glyphs/Familyname.glyphs
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.parser:Parsing .glyphs file
INFO:glyphsLib.builder:Running 'align_alternate_layers' transformation
INFO:glyphsLib.builder:Running 'propagate_all_anchors' transformation
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern1.dcaron found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern2.punctuation found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern1.dcaron found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern2.punctuation found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern1.dcaron found in kerning rules.
WARNING:glyphsLib.builder.builders.UFOBuilder:Non-existent glyph class public.kern2.punctuation found in kerning rules.
INFO:fontmake.font_project:Loading 3 DesignSpace source UFOs
INFO:fontmake.font_project:Building variable fonts variable_ttf/Familyname-VF.ttf
INFO:ufo2ft:Features are compatible across masters; building later
INFO:ufo2ft:Pre-processing glyphs
INFO:ufo2ft.filters:Running interpolatable SkipExportGlyphsIFilter
INFO:ufo2ft.filters:Running CornerComponentsFilter on Familyname-Extralight
fontmake: Error: In 'Familyname/sources/glyphs/Familyname.glyphs' -> 'master_ufo/Familyname.designspace': Generating fonts from Designspace failed: 'origin'

After checking all the font info a few times, I eventually realized it was failing on the CornerComponentsFilter step. So, I made a Glyphs filter to find glyphs where "Has Corners" is true.

This showed me that one glyph had a corner component, even though this was seemingly empty. Once I removed that in all layers, the fontmake build worked without issue.

Conclusion

Potentially, FontMake could give a more obvious error message here, like Generating fonts from Designspace failed: a corner component cannot be resolved in glyph /z. I’m not sure how feasible this is, though.

I’m also okay if this issue is closed, and just wanted it here for future searches by others or by myself. :)

Thanks!