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)