The implementation of [GHC proposal #409](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0409-exportable-named-default.rst#25rules-for-disambiguation-at-the-use-site) has brought with it a change in the way GHC defaults type variables. GHC is now less conservative: it will allow defaulting type variables, even when they appear as part of non-standard typeclasses. It used to be (as per [the Haskell report](https://www.haskell.org/onlinereport/decls.html#default-decls)) that any type variable that appeared in a non-standard constraint would not be defaultable. That restriction has now been lifted (since !11853 landed, in GHC 9.12), but this has not been documented in the release notes. The `NamedDefaults` section of the user's guide does explain these new defaulting rules, but not that they apply unconditionally whether `NamedDefaults` is enabled or not.