basic_iterator comparison operators are not symmetric, thus not equality comparable

[slymz]

Here is the issue in detail: https://godbolt.org/z/Ehsc36

In summary:

 bool b1{constantIter == mutableIter};  // ok: expected goodness.
 bool b2{mutableIter == constantIter};  // fails to compile

where constantIter and mutableIter is otherwise compatible iterators into const and non-const views of the same range, respectively.

This can be reproduced with any other custom view adaptor, where the adaptor allows for conversion, and provide correct assignment semantics, but fail to provide symmetric comparison. E.g.: https://godbolt.org/z/19qGGM

For context, I ran into this issue while upgrading our in-house Boost.Range (v2) iterators and ranges to range-v3. This was a showstopper. Here is the same example as above with Boost.Range fwiw: https://godbolt.org/z/MKKvWY

Is there a workaround?

[JohelEGP]

Using C++20, with its rewriting of comparison operators, is a workaround.