Fix for #2293: allow more than one lookahead glyph/class in contextual positioning with "value at end" (#2294)

* add test that fails for #2293

* fixing #2293: rewrite of contextual positioning logic, ensure len(suffix) > 1 yields the correct result; checking more edge cases and raising errors inspired by makeotf

* test error cases

* only check when we actually have a value

* catch one more case that makeotf errors on and we didn't

Lib/fontTools/feaLib/parser.py

@@ -473,14 +473,38 @@ class Parser(object):
             assert lookups == []
             return ([], prefix, [None] * len(prefix), values, [], hasMarks)
         else:
-            assert not any(values[: len(prefix)]), values
-            format1 = values[len(prefix) :][: len(glyphs)]
-            format2 = values[(len(prefix) + len(glyphs)) :][: len(suffix)]
-            values = (
-                format2
-                if format2 and isinstance(format2[0], self.ast.ValueRecord)
-                else format1
-            )
+            if any(values[: len(prefix)]):
+                raise FeatureLibError(
+                    "Positioning cannot be applied in the bactrack glyph sequence, "
+                    "before the marked glyph sequence.",
+                    self.cur_token_location_
+                )
+            marked_values = values[len(prefix) : len(prefix) + len(glyphs)]
+            if any(marked_values):
+                if any(values[len(prefix) + len(glyphs) :]):
+                    raise FeatureLibError(
+                        "Positioning values are allowed only in the marked glyph "
+                        "sequence, or after the final glyph node when only one glyph "
+                        "node is marked.",
+                        self.cur_token_location_
+                    )
+                values = marked_values
+            elif values and values[-1]:
+                if len(glyphs) > 1 or any(values[:-1]):
+                    raise FeatureLibError(
+                        "Positioning values are allowed only in the marked glyph "
+                        "sequence, or after the final glyph node when only one glyph "
+                        "node is marked.",
+                        self.cur_token_location_
+                    )
+                values = values[-1:]
+            elif any(values):
+                raise FeatureLibError(
+                    "Positioning values are allowed only in the marked glyph "
+                    "sequence, or after the final glyph node when only one glyph "
+                    "node is marked.",
+                    self.cur_token_location_
+                )
             return (prefix, glyphs, lookups, values, suffix, hasMarks)
 
     def parse_chain_context_(self):

Tests/feaLib/parser_test.py

@@ -870,6 +870,41 @@ class ParserTest(unittest.TestCase):
         self.assertEqual(glyphstr(pos.prefix), "[A B]")
         self.assertEqual(glyphstr(pos.suffix), "comma")
 
+    def test_gpos_type_1_chained_special_kern_format_valuerecord_format_b_bug2293(self):
+        # https://github.com/fonttools/fonttools/issues/2293
+        doc = self.parse("feature kern {pos [A B] [T Y]' comma a <0 0 0 0>;} kern;")
+        pos = doc.statements[0].statements[0]
+        self.assertIsInstance(pos, ast.SinglePosStatement)
+        [(glyphs, value)] = pos.pos
+        self.assertEqual(glyphstr([glyphs]), "[T Y]")
+        self.assertEqual(value.asFea(), "<0 0 0 0>")
+        self.assertEqual(glyphstr(pos.prefix), "[A B]")
+        self.assertEqual(glyphstr(pos.suffix), "comma a")
+
+    def test_gpos_type_1_chained_exception1(self):
+        with self.assertRaisesRegex(FeatureLibError, "Positioning values are allowed"):
+            doc = self.parse("feature kern {"
+                             "    pos [A B]' [T Y]' comma a <0 0 0 0>;"
+                             "} kern;")
+
+    def test_gpos_type_1_chained_exception2(self):
+        with self.assertRaisesRegex(FeatureLibError, "Positioning values are allowed"):
+            doc = self.parse("feature kern {"
+                             "    pos [A B]' <0 0 0 0> [T Y]' comma a <0 0 0 0>;"
+                             "} kern;")
+
+    def test_gpos_type_1_chained_exception3(self):
+        with self.assertRaisesRegex(FeatureLibError, "Positioning cannot be applied"):
+            doc = self.parse("feature kern {"
+                             "    pos [A B] <0 0 0 0> [T Y]' comma a <0 0 0 0>;"
+                             "} kern;")
+
+    def test_gpos_type_1_chained_exception4(self):
+        with self.assertRaisesRegex(FeatureLibError, "Positioning values are allowed"):
+            doc = self.parse("feature kern {"
+                             "    pos a' b c 123 d;"
+                             "} kern;")
+
     def test_gpos_type_2_format_a(self):
         doc = self.parse("feature kern {"
                          "    pos [T V] -60 [a b c] <1 2 3 4>;"