[name] Instances should only reuse name ID 2 or 17 if they are at the default location across all axes #3825
Description
Currently, the code tries to reuse name ID 2 or 17 for the "Regular" variable font instance here:
fonttools/Lib/fontTools/varLib/__init__.py
Lines 133 to 135 in e5c9b1e
The OT spec says this, though, emphasis mine:
The subfamilyNameID field provides a name ID that can be used to obtain strings from the 'name' table that can be treated as equivalent to name ID 17 (typographic subfamily) strings for the given instance. Values of 2 or 17 can be used; otherwise, values must be greater than 255 and less than 32768. The values 2 or 17 should only be used if the named instance corresponds to the font’s default instance.
Imagine a Designspace where the default weight is a Medium (wght=500) and there is a Regular instance at wght=400. The Regular instance should not use name ID 2 or 17 but something >= 255.
This is probably an irrelevant violation of the spec, but there are tools out there that check for it...
(Thanks to @Hoolean for identifying the issue internally.)