natvis is failing to read conditional with variables with Eigen

I am running Visual Studio 2017 15.9.5
I am trying to make an addition to a natvis for the Eigen library available here

I am basically making a variant of: <Type Name="Eigen::Matrix&lt;*,-1,-1,*,*,*&gt;"> for Eigen::Map.
I used the code already in place in the natvis as a platform:

  <Type Name="Eigen::Map&lt;Eigen::Matrix&lt;*,-1,-1,*,*,*&gt;,*,*&gt;">
    <DisplayString Condition="m_data == 0">empty</DisplayString>
    <DisplayString Condition="m_data != 0">Map[{m_rows.m_value}, {m_cols.m_value}] (dynamic matrix)</DisplayString>
    <Expand>
      <ArrayItems Condition="Flags%2"> <!-- row major layout -->
        <Rank>2</Rank>
        <Size>$i==0 ? m_rows.m_value : m_cols.m_value</Size>
        <ValuePointer>m_data</ValuePointer>
      </ArrayItems>
      <ArrayItems Condition="!(Flags%2)"> <!-- column major layout -->
        <Direction>Backward</Direction>
        <Rank>2</Rank>
        <Size>$i==0 ? m_rows.m_value : m_cols.m_value</Size>
        <ValuePointer>m_data</ValuePointer>
      </ArrayItems>
    </Expand>
  </Type>

Using trial and error, I know that my issue is with the line (in both locations):

<Size>$i==0 ? m_rows.m_value : m_cols.m_value</Size>

This is because if I replace either m_rows.m_value or m_cols.m_value with some constant integer, then it runs fine but not the way I want it to run, of course.

How do I get this to work?
I also have a stackoverflow questions here.