Nobody replied :( I found a much simpler way to do this; it's still a design change, but it's so easy to describe and implement that I decided to adopt it. See comment:14 of #12923. And Phab:D2822 Simon From: Simon Peyton Jones Sent: 03 February 2017 17:38 To: ghc-steering-committee@haskell.org Cc: Simon Peyton Jones Subject: Defaulting rules Dear GHC steering committee Advice needed. * Some time ago amindfv submitted https://ghc.haskell.org/trac/ghc/ticket/12923. He offered a patch 8 weeks ago. * There wasn't much discussion about the proposed change, nor did anyone ask him to undertake the GHC proposals process * It's a small change, but it is a user-facing one * The design isn't obvious to me; see comment:12. * We could change it now and change it again later; it's a corner case * It matters to him and his customers. So, should we let it in? It's about the defaulting mechanism. With an unsolved constraint (C alpha) where alpha is otherwise unconstrained, GHC will try a series of types to fill in alpha. We extended the rules with ExtendedDefaultRuleshttps://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#ty.... He wants a further extension. I hate slipping things in late, actually after the deadline; but the fault is partly ours for not proactively warning people with open patches. Opinions? Simon