fix for failing validation test, add syntax check in parser

Author: PhilippImhof

classes/local/answer_parser.php

@@ -69,7 +69,8 @@ public function __construct($tokenlist, array $knownvariables = [], bool $caretm
         // If we only have one single token and it is an empty string, we set it to the $EMPTY token.
         $firsttoken = reset($tokenlist);
         if (count($tokenlist) === 1 && $firsttoken->value === '') {
-            $tokenlist[0] = new token(token::EMPTY, '$EMPTY', $firsttoken->row, $firsttoken->column);
+            // FIXME: temporarily disabling this
+            // $tokenlist[0] = new token(token::EMPTY, '$EMPTY', $firsttoken->row, $firsttoken->column);
         }
 
         // Once this is done, we can parse the expression normally.

classes/local/parser.php

@@ -364,6 +364,17 @@ private function parse_general_expression(?int $stopat = null): expression {
                 $this->die(get_string('error_invalidrangesep', 'qtype_formulas'), $nexttoken);
             }
 
+            // We do not allow to a closing paren immediately after an opening paren of the same type.
+            $openingparen = ($type & token::ANY_OPENING_PAREN);
+            $nextclosing = ($nexttype & token::ANY_CLOSING_PAREN);
+            if ($openingparen && $nextclosing) {
+                $a = (object)[
+                    'open' => $value,
+                    'close' => $nextvalue,
+                ];
+                $this->die(get_string('error_emptyparens', 'qtype_formulas', $a), $nexttoken);
+            }
+
             // If we're one token away from the end of the statement, we just read and discard the end-of-statement marker.
             if ($nexttype === token::END_OF_STATEMENT || $nexttype === token::END_GROUP) {
                 $this->read_next();

lang/en/qtype_formulas.php

@@ -141,6 +141,7 @@
 $string['error_distribution_outcomes'] = '{$a} expects the number of successful outcomes to be a non-negative integer.';
 $string['error_distribution_tries'] = '{$a} expects the number of tries to be a non-negative integer.';
 $string['error_divzero'] = 'Division by zero is not defined.';
+$string['error_emptyparens'] = 'Parse error: closing {$a->close} immediately after opening {$a->open}.';
 $string['error_emptyrange'] = 'Evaluation error: range from {$a->start} to {$a->end} with step {$a->step} will be empty.';
 $string['error_emptystack'] = 'Evaluation error: empty stack - did you pass enough arguments for the function or operator?';
 $string['error_evaluate_invocation'] = 'Bad invocation of {$a}.';