[feaLib] Disallow lookup blocks inside feature aalt

The new behavior matches how makeotf v2.0.90 interprets
the OpenType Feature File Specification.

Lib/fontTools/feaLib/builder.py

@@ -364,6 +364,11 @@ class Builder(object):
         if name in self.named_lookups_:
             raise FeatureLibError(
                 'Lookup "%s" has already been defined' % name, location)
+        if self.cur_feature_name_ == "aalt":
+            raise FeatureLibError(
+                "Lookup blocks cannot be placed inside 'aalt' features; "
+                "move it out, and then refer to it with a lookup statement",
+                location)
         self.cur_lookup_name_ = name
         self.named_lookups_[name] = None
         self.cur_lookup_ = None

Lib/fontTools/feaLib/builder_test.py

@@ -329,6 +329,12 @@ class BuilderTest(unittest.TestCase):
             "    sub A from [A.alt1 A.alt2];"
             "} foo;")
 
+    def test_lookup_inside_feature_aalt(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Lookup blocks cannot be placed inside 'aalt' features",
+            self.build, "feature aalt {lookup L {} L;} aalt;")
+
 
 def generate_feature_file_test(name):
     return lambda self: self.check_feature_file(name)
@@ -339,4 +345,3 @@ for name in BuilderTest.TEST_FEATURE_FILES:
 
 if __name__ == "__main__":
     unittest.main()
-