[WWcvs] pg: Make sure the previous answer is of the same MathObject class as the object being checked (so the comparison is done by the

[dpvc v. a. <we...@ma...>]

Log Message:
-----------
Make sure the previous answer is of the same MathObject class as the
object being checked (so the comparison is done by the right object).
This came up with the ImplicitEquation object).

Modified Files:
--------------
    pg/lib/Value:
        AnswerChecker.pm

Revision Data
-------------
Index: AnswerChecker.pm
===================================================================
RCS file: /webwork/cvs/system/pg/lib/Value/AnswerChecker.pm,v
retrieving revision 1.116
retrieving revision 1.117
diff -Llib/Value/AnswerChecker.pm -Llib/Value/AnswerChecker.pm -u -r1.116 -r1.117
--- lib/Value/AnswerChecker.pm
+++ lib/Value/AnswerChecker.pm
@@ -1647,7 +1647,8 @@
   return $ans if $ans->{ans_message}; # don't overwrite other messages
   $ans->{prev_formula} = Parser::Formula($self->{context},$ans->{prev_ans});
   if (defined($ans->{prev_formula}) && defined($ans->{student_formula})) {
-    $ans->{prev_equals_current} = Value::cmp_compare($ans->{student_formula},$ans->{prev_formula},{});
+    my $prev = eval {$self->promote($ans->{prev_formula})}; break unless defined($prev);
+    $ans->{prev_equals_current} = Value::cmp_compare($prev,$ans->{student_formula},{});
     if (   !$ans->{isPreview}                                 # not preview mode
 	and $ans->{prev_equals_current}                       # equivalent
 	and $ans->{prev_ans} ne $ans->{original_student_ans}) # but not identical