Specializing functions with implicit parameters

[Alexis King]

> On Mar 14, 2020, at 20:03, Sandy Maguire <sandy at sandymaguire.me> wrote:
> 
> What GHC are you testing against? I suspect https://gitlab.haskell.org/ghc/ghc/merge_requests/668 <https://gitlab.haskell.org/ghc/ghc/merge_requests/668> will fix this.

I’ve tested against HEAD. I think the change you link is helpful, but it doesn’t quite get there: the usage gets dumped before specHeader even gets a chance to look at the call. The relevant bit of code is here:

https://gitlab.haskell.org/ghc/ghc/blob/1de3ab4a147eeb0b34b24a3c0e91f174e6e5cb79/compiler/specialise/Specialise.hs#L2274-2302 <https://gitlab.haskell.org/ghc/ghc/blob/1de3ab4a147eeb0b34b24a3c0e91f174e6e5cb79/compiler/specialise/Specialise.hs#L2274-2302>

Specifically, this line seals the deal:

    ClassPred cls _ -> not (isIPClass cls)  -- Superclasses can't be IPs

So maybe the right fix is just to change the role of type_determines_value so that it turns SpecDicts into UnspecArgs, and then with your change everything would just happily work out.
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