see comments at the end of #1792
RegressionModel does no implicit constant detection.
The data handling code in base only detects explicit constant (and has a bug for multiple zero variance columns
TODO: merge the use of RegressionModel._has_constant, into base.data.ModelData._handle_constant
Note: we will end up with possible repeated matrix_rank calculations that users can avoid by explicitly providing hasconst in the model creation (__init__).
I have the PR almost finished and will push tonight.
changed in #1797