[feaLib] Reject script dflt and language DFLT

These are invalid casing variants of the frequently used
`script DFLT` and `language dflt`. Adobe's makeotf tool
does a similar check.

Lib/fontTools/feaLib/parser.py

@@ -166,7 +166,8 @@ class Parser(object):
 
     def parse_language_(self):
         assert self.is_cur_keyword_("language")
-        location, language = self.cur_token_location_, self.expect_tag_()
+        location = self.cur_token_location_
+        language = self.expect_language_tag_()
         include_default, required = (True, False)
         if self.next_token_ in {"exclude_dflt", "include_dflt"}:
             include_default = (self.expect_name_() == "include_dflt")
@@ -201,7 +202,7 @@ class Parser(object):
 
     def parse_script_(self):
         assert self.is_cur_keyword_("script")
-        location, script = self.cur_token_location_, self.expect_tag_()
+        location, script = self.cur_token_location_, self.expect_script_tag_()
         self.expect_symbol_(";")
         return ast.ScriptStatement(location, script)
 
@@ -294,7 +295,8 @@ class Parser(object):
     def parse_languagesystem_(self):
         assert self.cur_token_ == "languagesystem"
         location = self.cur_token_location_
-        script, language = self.expect_tag_(), self.expect_tag_()
+        script = self.expect_script_tag_()
+        language = self.expect_language_tag_()
         self.expect_symbol_(";")
         if script == "DFLT" and language != "dflt":
             raise FeatureLibError(
@@ -371,6 +373,22 @@ class Parser(object):
                                   self.cur_token_location_)
         return (self.cur_token_ + "    ")[:4]
 
+    def expect_script_tag_(self):
+        tag = self.expect_tag_()
+        if tag == "dflt":
+            raise FeatureLibError(
+                '"dflt" is not a valid script tag; use "DFLT" instead',
+                self.cur_token_location_)
+        return tag
+
+    def expect_language_tag_(self):
+        tag = self.expect_tag_()
+        if tag == "DFLT":
+            raise FeatureLibError(
+                '"DFLT" is not a valid language tag; use "dflt" instead',
+                self.cur_token_location_)
+        return tag
+
     def expect_symbol_(self, symbol):
         self.advance_lexer_()
         if self.cur_token_type_ is Lexer.SYMBOL and self.cur_token_ == symbol:

Lib/fontTools/feaLib/parser_test.py

@@ -199,6 +199,12 @@ class ParserTest(unittest.TestCase):
         self.assertTrue(s.include_default)
         self.assertTrue(s.required)
 
+    def test_language_DFLT(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            '"DFLT" is not a valid language tag; use "dflt" instead',
+            self.parse, "feature test { language DFLT; } test;")
+
     def test_lookup_block(self):
         [lookup] = self.parse("lookup Ligatures {} Ligatures;").statements
         self.assertEqual(lookup.name, "Ligatures")
@@ -256,6 +262,12 @@ class ParserTest(unittest.TestCase):
         self.assertEqual(type(s), ast.ScriptStatement)
         self.assertEqual(s.script, "cyrl")
 
+    def test_script_dflt(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            '"dflt" is not a valid script tag; use "DFLT" instead',
+            self.parse, "feature test {script dflt;} test;")
+
     def test_substitute_single_format_a(self):  # GSUB LookupType 1
         doc = self.parse("feature smcp {substitute a by a.sc;} smcp;")
         sub = doc.statements[0].statements[0]
@@ -398,6 +410,14 @@ class ParserTest(unittest.TestCase):
             FeatureLibError,
             'For script "DFLT", the language must be "dflt"',
             self.parse, "languagesystem DFLT DEU;")
+        self.assertRaisesRegex(
+            FeatureLibError,
+            '"dflt" is not a valid script tag; use "DFLT" instead',
+            self.parse, "languagesystem dflt dflt;")
+        self.assertRaisesRegex(
+            FeatureLibError,
+            '"DFLT" is not a valid language tag; use "dflt" instead',
+            self.parse, "languagesystem latn DFLT;")
         self.assertRaisesRegex(
             FeatureLibError, "Expected ';'",
             self.parse, "languagesystem latn DEU")