[Bug #20784] Fix incomplete character syntax followed by EOF

Author: tompng

parse.y

@@ -10118,7 +10118,7 @@ parse_qmark(struct parser_params *p, int space_seen)
             enc = rb_utf8_encoding();
             tokadd_utf8(p, &enc, -1, 0, 0);
         }
-        else if (!ISASCII(c = peekc(p))) {
+        else if (!ISASCII(c = peekc(p)) && c != -1) {
             nextc(p);
             if (tokadd_mbchar(p, c) == -1) return 0;
         }

test/ripper/test_scanner_events.rb

@@ -951,6 +951,10 @@ def test_CHAR
     assert_equal ["?\\u{41}"],
                  scan('CHAR', "?\\u{41}")
 
+    err = nil
+    assert_equal [], scan('CHAR', '?\\') {|*e| err = e}
+    assert_equal([:on_parse_error, "Invalid escape character syntax", "?\\"], err)
+
     err = nil
     assert_equal [], scan('CHAR', '?\\M ') {|*e| err = e}
     assert_equal([:on_parse_error, "Invalid escape character syntax", "?\\M "], err)