When a discriminator is evaluated, that evaluation occurs in a special mode so that you can write this sort of thing:

<element ref="foo">

    <dfdl:discriminator>{ ../foo/bar eq 2 }</dfdl:discriminator>

</element>

Outside the context of a discriminator, if ../foo/bar doesn't exist that's a runtime SDE. Fatal.

Inside the context of a discriminator, if that doesn't exist, it's a PE, and the disriminator simply is false. But it's not fatal.

The bug is that Daffodil is always issuing a runtime SDE for this. We need this behavior for discriinators.