[feaLib] Don't set language when script is unset

Before, if someone tried to set the language before setting the script
a None/language language system would be created (with actual tag
"None" stored in the feature table). This defaults to tag DFLT and
fails when a non-dflt language is set for DFLT, since that's illegal.

Lib/fontTools/feaLib/builder.py

@@ -561,6 +561,7 @@ class Builder(object):
 
     def start_feature(self, location, name):
         self.language_systems = self.get_default_language_systems_()
+        self.script_ = 'DFLT'
         self.cur_lookup_ = None
         self.cur_feature_name_ = name
         self.lookupflag_ = 0
@@ -613,6 +614,9 @@ class Builder(object):
             raise FeatureLibError(
                 "Language statements are not allowed "
                 "within \"feature %s\"" % self.cur_feature_name_, location)
+        if language != 'dflt' and self.script_ == 'DFLT':
+            raise FeatureLibError("Need non-DFLT script when using non-dflt "
+                                  "language (was: \"%s\")" % language, location)
         self.cur_lookup_ = None
 
         key = (self.script_, language, self.cur_feature_name_)

Lib/fontTools/feaLib/builder_test.py

@@ -255,6 +255,12 @@ class BuilderTest(unittest.TestCase):
                              include_default=True, required=False)
         self.assertEqual(builder.language_systems,
                          {('cyrl', 'BGR ')})
+        builder.start_feature(location=None, name='test2')
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Need non-DFLT script when using non-dflt language "
+            "\(was: \"FRA \"\)",
+            builder.set_language, None, 'FRA ', True, False)
 
     def test_language_in_aalt_feature(self):
         self.assertRaisesRegex(
@@ -281,6 +287,7 @@ class BuilderTest(unittest.TestCase):
             "    substitute [a-z] by [A.sc-Z.sc];"
             "} scmp;"
             "feature test {"
+            "    script latn;"
             "    language FRA required;"
             "    substitute [a-z] by [A.sc-Z.sc];"
             "} test;")

Lib/fontTools/feaLib/testdata/language_required.fea

@@ -12,6 +12,7 @@ feature hlig {
 } hlig;
 
 feature liga {
+  script latn;
   language ITA exclude_dflt required;
   sub I T A by I.sc;
 } liga;