Feedback for overloading of BitOr operator for match statements

[Otto-AA]

Some feedback that may be useful in future design decisions. I don't think this could be changed without a new major release which probably is not worth it.

The MatchOrElement uses a BitOr element to denote the separator. For the mutation testing library I contribute to, this is an unexpected overloading/reuse of the BitOr operator. At least in our usage, it would be better to have a different node class for this (e.g. MatchOrSeparator) to easily differentiate from the one used in expressions.

Previously, to create potential logic bugs, we replaced all operators with their opposite. So a = would be replaced with !=, + with -, etc. This seems to work well for everything now, except for the | to & mapping. The | to & mapping fails because it would map the match case case A() | B() to case A() & B() (which apparently gets rendered as case A()B() though I did not investigate this).

To fix this, I had to change the logic to only replace BitOr nodes within a BinaryOperation node. The change was not too big, so in our setup it was rather quick to fix this, but still unexpected behaviour to me.

I hope this feedback is helpful for you. The corresponding bug report at the repo: boxed/mutmut#402