GSUB table overflow with mega substitutions

We do some large icon fonts, which we build with fontmake.

When we try to use fontc, it fails with a GSUB table overflow. Which is not as shocking as it would be with a normal font, as we have about 15K substitutions. So, yes, it is a bit large. But our fontmake build does not fail.

I don’t *think* the existing fontmake uses a subtable break to avoid this... a decompiled ttx file does not show the word “subtable” anywhere, which IIRC I would see if there was one in there.

I am wondering whether fontc is for some reason generating a larger GSUB than fontmake?
Or does fontmake automatically fix the lookup, in some way I am missing, while fontc does not?

Start by reproducing the GSUB table overflow with the reported large icon font using fontc, then compare its output with the successful fontmake build and decompiled ttx output. Determine whether the difference is caused by missing subtable handling or larger generated data; done means identifying the cause and defining a verified fix or clear limitation.

Written by the indexing model from the issue text.