[instancer] Allow null ConditionSet

Lib/fontTools/varLib/instancer/featureVars.py

@@ -8,7 +8,10 @@
 
 def _featureVariationRecordIsUnique(rec, seen):
     conditionSet = []
-    for cond in rec.ConditionSet.ConditionTable:
+    conditionSets = (
+        rec.ConditionSet.ConditionTable if rec.ConditionSet is not None else []
+    )
+    for cond in conditionSets:
         if cond.Format != 1:
             # can't tell whether this is duplicate, assume is unique
             return True
@@ -54,6 +57,10 @@ def _instantiateFeatureVariationRecord(
     from fontTools.varLib.instancer import NormalizedAxisTripleAndDistances
 
     default_triple = NormalizedAxisTripleAndDistances(-1, 0, +1)
+    if record.ConditionSet is None:
+        record.ConditionSet = ot.ConditionSet()
+        record.ConditionSet.ConditionTable = []
+        record.ConditionSet.ConditionCount = 0
     for i, condition in enumerate(record.ConditionSet.ConditionTable):
         if condition.Format == 1:
             axisIdx = condition.AxisIndex
@@ -100,6 +107,8 @@ def _instantiateFeatureVariationRecord(
 
     if newConditions is not None and shouldKeep:
         record.ConditionSet.ConditionTable = newConditions
+        if not newConditions:
+            record.ConditionSet = None
         shouldKeep = True
     else:
         shouldKeep = False

Tests/varLib/instancer/instancer_test.py

@@ -1595,7 +1595,8 @@ def test_varComposite(self):
 
 def _conditionSetAsDict(conditionSet, axisOrder):
     result = {}
-    for cond in conditionSet.ConditionTable:
+    conditionSets = conditionSet.ConditionTable if conditionSet is not None else []
+    for cond in conditionSets:
         assert cond.Format == 1
         axisTag = axisOrder[cond.AxisIndex]
         result[axisTag] = (cond.FilterRangeMinValue, cond.FilterRangeMaxValue)
@@ -1821,6 +1822,24 @@ def test_full_instance(self, location, appliedSubs):
         else:
             assert not gsub.FeatureList.FeatureRecord
 
+    def test_null_conditionset(self):
+        # A null ConditionSet offset should be treated like an empty ConditionTable, i.e.
+        # all contexts are matched; see https://github.com/fonttools/fonttools/issues/3211
+        font = makeFeatureVarsFont(
+            [([{"wght": (-1.0, 1.0)}], {"uni0024": "uni0024.nostroke"})]
+        )
+        gsub = font["GSUB"].table
+        gsub.FeatureVariations.FeatureVariationRecord[0].ConditionSet = None
+
+        location = instancer.NormalizedAxisLimits({"wght": 0.5})
+        instancer.instantiateFeatureVariations(font, location)
+
+        assert not hasattr(gsub, "FeatureVariations")
+        assert gsub.Version == 0x00010000
+
+        lookupIndices = gsub.FeatureList.FeatureRecord[0].Feature.LookupListIndex
+        assert _getSubstitutions(gsub, lookupIndices) == {"uni0024": "uni0024.nostroke"}
+
     def test_unsupported_condition_format(self, caplog):
         font = makeFeatureVarsFont(
             [