[feaLib] Check lookup index in chaining lookups

Raise if lookup_index is None, which would happen only of the lookup
referenced belongs to the wrong table. The error message needs to be less
cryptic, though.

Fixes https://github.com/fonttools/fonttools/issues/1896

Lib/fontTools/feaLib/builder.py

@@ -1263,6 +1263,9 @@ class ChainContextPosBuilder(LookupBuilder):
             st.PosLookupRecord = []
             for sequenceIndex, l in enumerate(lookups):
                 if l is not None:
+                    if l.lookup_index is None:
+                        raise FeatureLibError('Missing lookup index',
+                            self.location)
                     rec = otTables.PosLookupRecord()
                     rec.SequenceIndex = sequenceIndex
                     rec.LookupListIndex = l.lookup_index
@@ -1310,6 +1313,9 @@ class ChainContextSubstBuilder(LookupBuilder):
             st.SubstLookupRecord = []
             for sequenceIndex, l in enumerate(lookups):
                 if l is not None:
+                    if l.lookup_index is None:
+                        raise FeatureLibError('Missing lookup index',
+                            self.location)
                     rec = otTables.SubstLookupRecord()
                     rec.SequenceIndex = sequenceIndex
                     rec.LookupListIndex = l.lookup_index

Tests/feaLib/builder_test.py

@@ -430,6 +430,28 @@ class BuilderTest(unittest.TestCase):
             "Lookup blocks cannot be placed inside 'aalt' features",
             self.build, "feature aalt {lookup L {} L;} aalt;")
 
+    def test_chain_subst_refrences_GPOS_looup(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Missing lookup index",
+            self.build,
+            "lookup dummy { pos a 50; } dummy;"
+            "feature test {"
+            "    sub a' lookup dummy b;"
+            "} test;"
+        )
+
+    def test_chain_pos_refrences_GSUB_looup(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Missing lookup index",
+            self.build,
+            "lookup dummy { sub a by A; } dummy;"
+            "feature test {"
+            "    pos a' lookup dummy b;"
+            "} test;"
+        )
+
     def test_extensions(self):
         class ast_BaseClass(ast.MarkClass):
             def asFea(self, indent=""):