postscriptBlueValues are not written for UFOs extracted from a Glyphs file if the overshoot value has wrong polarity

[kontur]

Extracting UFO masters/instances from a Glyphs file where some overshoots are defined with the wrong polarity the exported UFOs are missing postscriptBlueValues entirely.

E.g. these master metrics values would cause this (rightly so, I suppose):

Cap Height 700 -15 (should be positive overshoot for zones above 0)
Descender -250 15 (should be negative overshoot for zones below 0)

Instead of silently dropping the fontinfo.plist postscriptBlueValues entry, I'd expect a warning. I couldn't pin point where exactly in the compile stack this assertion kicks in.

Attached a file where fontmake -g Test-PostscriptBlueValues-Fails.glyphs -o ufo will result in two masters, the master_ufo/TestfontPostscriptBlueValueFailing-Black.ufo/fontinfo.plist correctly with <key>postscriptBlueValues</key>

And master_ufo/TestfontPostscriptBlueValueFailing-Thin.ufo/fontinfo.plist missing the entry. (The same applies to the instances when ran with -i).

I'd expect:

• a warning or, preferably, failing with a descriptive error
• other, valid, zones to be compiled and resulting in a postscriptBlueValues entry (just one erroneous overshoot will cause all zones to be dropped)

Test-PostscriptBlueValues-Fails.glyphs.zip