Hello,
Using Eigen::Matrix for the state_type [0] in odeint worked until a
change in Eigen 3.3. Issue #194 [1] tracks the history of the issue,
including a recent patch [2] by one of the odeint maintainers to fix
the issue.

The patch [2] depends on the internal namespace of Eigen. Does this
seem reasonable or is there any guidance from an Eigen expert on how
this patch might be made to work without depending on Eigen::internal?

Sincerely,
Luke

[0] https://www.boost.org/doc/libs/1_69_0/l ... tions.html
[1] https://github.com/headmyshoulder/odeint-v2/issues/194
[2] https://github.com/headmyshoulder/odeint-v2/pull/237