I’m catching up here.

Let me share a few thoughts:

• I really want something like this (both for better OverloadedList support, and because it’s super useful in tests)
• I’m rather unsure what to think about regarding the non-total priority (the fact that you can have default C (A, B); default D (B, A) and need to default a variable x with (C x, D x)). This sounds like something that must at least be specified. Am I correct that it isn’t?
• There is no point in separating NamedDefaults and ExportedDefaults in two extensions
• Regarding imports: in a first approximation explicit imports are useless. Having implicit imports lets me, for instance, define default IsList ([]) in the prelude, and then turn OverloadedList on in GHC2023 and not break existing programs. Yay. Explicit default imports just save me one copy-paste. Note by the way that if GHC2023 is the target, then the extension to import defaults would also have to be included in GHC2023, so, basically, ImportedDefaults is practically useless, and we should just import defaults (I don’t think a warning would make much sense for a normal default behaviour; I agree with Simon that this is not worse than importing overloaded instances).

Still, there are some dark corners (I have pointed out one above, but I also find the exports and imports kind of difficult to wrap my head around). So I guess the conversation is not over quite yet.