New check: presence of GSUB5/GPOS7

[simoncozens]

Versions of fonttools >=4.14.0 (19 August 2020) perform an optimisation on chained contextual lookups, expressing GSUB6 as GSUB5 and GPOS8 and GPOS7 where possible (when there are no suffixes/prefixes for all rules in the lookup).

However, makeotf has never generated these lookup types and they are rare in practice. Perhaps before of this, Mac's CoreText shaper (and probably other environments) do not correctly interpret these lookup types, meaning that these lookups will be ignored by the shaper, and fonts containing these lookups will have unintended positioning and substitution errors.

I think this should not be a FAIL because the fonts are technically correct (it's totally Apple's fault), but it should be a very severe WARN! I'll work on a PR for it.

[felipesanches]

I think this should be a FAIL because even though the fonts "are technically correct (it's totally Apple's fault)", the problem exists and must be avoided.

[anthrotype]

should be a FAIL

not if a font targets non-apple platform. Unless there's a way to disable or demote severity of checks

[simoncozens]

It feels wrong to FAIL correct fonts.

[m4rc1e]

I'd like us to move away from failing fonts which are spec complaint but don't work well on certain platforms/software. By "fixing" the fonts, it just incentivises the platforms/software not to bother fixing the actual issue.

[bobh0303]

For the font developers, is there a simple way to turn off these fontTools optimizations? @simoncozens seems to hint at that in his tweet.

[devosb]

Thank you for finding this @simoncozens I am sorry to point out a typo, in the phrase GSUB6 as GSUB5 and GPOS8 and GPOS7 I think the second and should be as. This typo has now made it into the Font Bakery code as well.

[Fuzzypeg]

The warning text says "To fix this warning, rebuild the font with a recent version of fonttools."
I have installed the latest version of fonttools and still get the problem. I think the warning text may be misleading.