visitCallExpression() in ConstexprFolder calls unifyNode() on the argument, which can be a variableRef, which asks the "variable" parameter what its unifyNode() is, but it hasn't been set because we are doing this before the NameResolver or Checker has run.
Perhaps the solution is not to call unifyNode in ConstexprFolder because the only time we use ConstexprFolder is before the "variable" parameter has been set.
*** This bug has been marked as a duplicate of bug 188873 ***
This is done.
Migrated to https://github.com/gpuweb/WHLSL/issues/103