If I remember correctly, we added more aggressive expansion of superclasses in quantified-constraint givens. But it's all a bit surprising that it works, given that the superclass given will have a type family in an instance head, which is supposed to not happen (because inference would be fragile, not because there's anything unsound). Richard

On Sep 20, 2022, at 5:07 PM, Li-yao Xia wrote:

Dear Café,

There's a trick [1] involving quantified constraints (and usually type families) where a function has a quantified constraint (forall a. CF a) and which is explicitly instantiated with a well-placed type annotation (_ :: CF x => ...). Since GHC 9.2, this trick works without that type annotation. How did GHC get smarter at instantiating this quantified constraint? Below is a minimized example which compiles on GHC 9.2.1 but not 9.0.1 (haven't tested 9.0.2), unless you uncomment the last line.

Cheers, Li-yao

[1]: an Iceland_jack trick https://gitlab.haskell.org/ghc/ghc/-/issues/14860#note_151394

{-# LANGUAGE TypeFamilies, FlexibleInstances, FlexibleContexts, QuantifiedConstraints, ScopedTypeVariables #-} module A where

type family F a class C a class C (F a) => CF a

f :: C (F a) => a f = undefined

g :: (forall a. CF a) => a g = f -- :: forall a. CF a => a -- needed until GHC 9.0.1 but not GHC 9.2.1 (and later)

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