From e0c49a1d4a802041ba90fff8bb8d1f991e2fe50f Mon Sep 17 00:00:00 2001
From: Khaled Hosny <khaledhosny@eglug.org>
Date: Mon, 27 Apr 2020 18:13:18 +0200
Subject: [PATCH] [feaLib] Improve exception message

---
 Lib/fontTools/feaLib/builder.py | 6 ++++--
 Tests/feaLib/builder_test.py    | 4 ++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/Lib/fontTools/feaLib/builder.py b/Lib/fontTools/feaLib/builder.py
index 4e7826d01..72156b8bb 100644
--- a/Lib/fontTools/feaLib/builder.py
+++ b/Lib/fontTools/feaLib/builder.py
@@ -1264,7 +1264,8 @@ class ChainContextPosBuilder(LookupBuilder):
             for sequenceIndex, l in enumerate(lookups):
                 if l is not None:
                     if l.lookup_index is None:
-                        raise FeatureLibError('Missing lookup index',
+                        raise FeatureLibError('Missing index of the specified '
+                            'lookup, might be a substitution lookup',
                             self.location)
                     rec = otTables.PosLookupRecord()
                     rec.SequenceIndex = sequenceIndex
@@ -1314,7 +1315,8 @@ class ChainContextSubstBuilder(LookupBuilder):
             for sequenceIndex, l in enumerate(lookups):
                 if l is not None:
                     if l.lookup_index is None:
-                        raise FeatureLibError('Missing lookup index',
+                        raise FeatureLibError('Missing index of the specified '
+                            'lookup, might be a positioning lookup',
                             self.location)
                     rec = otTables.SubstLookupRecord()
                     rec.SequenceIndex = sequenceIndex
diff --git a/Tests/feaLib/builder_test.py b/Tests/feaLib/builder_test.py
index a90bc91e6..3273ba618 100644
--- a/Tests/feaLib/builder_test.py
+++ b/Tests/feaLib/builder_test.py
@@ -433,7 +433,7 @@ class BuilderTest(unittest.TestCase):
     def test_chain_subst_refrences_GPOS_looup(self):
         self.assertRaisesRegex(
             FeatureLibError,
-            "Missing lookup index",
+            "Missing index of the specified lookup, might be a positioning lookup",
             self.build,
             "lookup dummy { pos a 50; } dummy;"
             "feature test {"
@@ -444,7 +444,7 @@ class BuilderTest(unittest.TestCase):
     def test_chain_pos_refrences_GSUB_looup(self):
         self.assertRaisesRegex(
             FeatureLibError,
-            "Missing lookup index",
+            "Missing index of the specified lookup, might be a substitution lookup",
             self.build,
             "lookup dummy { sub a by A; } dummy;"
             "feature test {"