feaLib: when name table not selected, skip building featureNames blocks

ufo2ft feature writer calls feaLib builder with tables=[GSUB] first, to run closure
over glyph substitutions; if the GSUB features contains 'featureNames' blocks, then
an AssertionError will be raised; in this case, we can simply skip building the
FeatureParams tables as we haven't build the name records they point to.

Lib/fontTools/feaLib/builder.py

@@ -290,10 +290,14 @@ class Builder(object):
             else:
                 params.SubfamilyNameID = 0
         elif tag in self.featureNames_:
-            assert tag in self.featureNames_ids_
-            params = otTables.FeatureParamsStylisticSet()
-            params.Version = 0
-            params.UINameID = self.featureNames_ids_[tag]
+            if not self.featureNames_ids_:
+                # name table wasn't selected among the tables to build; skip
+                pass
+            else:
+                assert tag in self.featureNames_ids_
+                params = otTables.FeatureParamsStylisticSet()
+                params.Version = 0
+                params.UINameID = self.featureNames_ids_[tag]
         elif tag in self.cv_parameters_:
             params = otTables.FeatureParamsCharacterVariants()
             params.Format = 0

Tests/feaLib/builder_test.py

@@ -524,6 +524,19 @@ class BuilderTest(unittest.TestCase):
             "} liga;"
         )
 
+    def test_skip_featureNames_if_no_name_table(self):
+        features = (
+            "feature ss01 {"
+            "    featureNames {"
+            '        name "ignored as we request to skip name table";'
+            "    };"
+            "    sub A by A.alt1;"
+            "} ss01;"
+        )
+        font = self.build(features, tables=["GSUB"])
+        self.assertIn("GSUB", font)
+        self.assertNotIn("name", font)
+
 
 def generate_feature_file_test(name):
     return lambda self: self.check_feature_file(name)