Prohibition in GSUB Multiple Substitution Lookup contradicts long-standing practice.

[drj11]

GSUB section 2.1 Multiple Subsitution says the following:

The use of multiple substitution for deletion of an input glyph is prohibited. The glyphCount value must always be greater than 0.

(The previous version of the OpenType specification says this too, it isn't new)

But a 0-length replacement sequence is exactly how the AFDKO Feature File compilation tool makeotf implements the rule to remove a glyph from the input: sub X by NULL compiles a Multiple Substition lookup with a 0-length replacement sequence (and i assume it always has done).

I therefore suggest that years of standing practice bless this implementation detail, and that the OpenType specification is changed. To something like:

Multiple substitution can be used to implement the deletion of an input glyph. When the glyphCount is 0 the input glyph is replaced with an empty sequence, effectively deleting it.

I have raised this point in the AFDKO repo too adobe-type-tools/afdko#1438

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Document Details

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• ID: 9c725801-0473-3105-d777-ddbaa087ad2d
• Version Independent ID: 9c725801-0473-3105-d777-ddbaa087ad2d
• Content: GSUB — Glyph Substitution Table (OpenType 1.9) - Typography
• Content Source: typographydocs/opentype/otspec190/gsub.md
• Product: typography
• GitHub Login: @PeterCon
• Microsoft Alias: alib

[simoncozens]

Duplicate of #707 and #673.

[drj11]

ahahahahahaaha