@bgamari I think what we (GHC developers, GHC Steering Committee, CLC) historically fail to recognise that stability is not just a technical, but also a social question. It’s too easy to take a defensive stance “this is a bug fix, not a breaking change”, “there is no such thing as absolute stability”, “GADTs are an experimental feature” or come up with a plausible technical justification why the breakage is for the greater good. I do exactly the same in my CLC hat.
Things however change when you are at the receiving end. At some point it does not matter how well motivated was the change or how many papers and committees support it. Enough is enough.