disable automatic GDEF generation when building variable fonts

how can we turn off automatic GDEF generation when building variable fonts?

I’ve tried --feature-writer None and/or --no-generate-GDEF – this works when generating a static font, but it does not work when generating a variable font.

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Context

fontmake's automatic GDEF table generation assumes that all glyphs containing anchors with names starting with an underscore are marks (non-spacing combining glyph, GDEF class 3). such glyphs are not always marks though; there are other use cases for anchors, such as building glyphs from parts; and there is an older convention to use for example spam (in the base glyph) and _spam (in the component glyph) to attach component to base. Having _spam in a glyph like /O (to attach it to some other glyph) does not mean it should be treated as a mark.

• simple test case

[BoldMonday]

You can circumvent this by adding your own GDEF table in the feature file:

table GDEF {
	GlyphClassDef
	[ a b c ... ],
	[ fi fl ... ],
	[ macroncomb tildecomb dotaccentcomb ... ],
	;
} GDEF;

https://github.com/adobe-type-tools/afdko/blob/develop/docs/OpenTypeFeatureFileSpecification.md#9.b

thanks Paul, this works indeed as a way to override fontmake's automatic GDEF — which is better than patching the buggy ttf using ttx, as I was doing before.

the issues remain though:

1. --feature-writer None is not turning off automatic feature generation for variable fonts, only for static fonts.
2. fontMake's anchor naming convention for GDEF generation clashes with older conventions used in glyph building tools (FL, RoboFab, glyphConstruction) – see also Error when parsing anchor names ending with underscore-number #1104 for another example.