[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) relaxed the rules for defaulting at use-sites. However, the implementation doesn't match the specification, as `GHC.Tc.Solver.Default.findDefaultableGroups` still considers any type variable that appears in a non-unary constraint to be non-defaultable. So we should either: 1. Remove that check entirely, or 2. Remove that check when `-XNamedDefaults` is enabled. Note that this check is already skipped when `-XExtendedDefaultRules` is enabled. (2) seems like the easier sell, but I still find it quite strange that enabling/disabling the extension would affect this case (non-unary constraints) when the rest of the logic (such as defaulting non-standard classes) is not gated at all (see my analysis in [this comment](https://gitlab.haskell.org/ghc/ghc/-/issues/25775#note_611005)). See also * #25775 * #25807