The test you added is fine. However, I was surprised that this PR was needed, as I had just recently used cross in my own code, and the type checkers didn't report an error. Turns out that pd.merge() types how as str, so would you mind making the change there as well to have it use MergeHow as the type, and then add a similar test?

PR for pandas: pandas-dev/pandas#48513

@alexc-yellowcanary Can you merge in current main, resolve conflicts, and push. In #355 we handled the issue of fixing the how argument in pd.merge(), but I'd like to use what you have here for MergeHow in there, or vice versa, dependent on which PR gets approved first.

This fix here isn't 100% correct. Need distinct joinhow and megehow types since the 4 core values are widely used, and cross is only for merge.

Test is now in #355

Closed in favor of #355