Hi, I have the following code that builds successfully with "stack build" under GHC 8.10.7 but fails to build under GHC 9.0.1: https://github.com/jchia/ip-bug The compiler is switched by commenting/uncommenting the compiler line in stack.yaml. GHC 9.0.1 gives me the error I paste at the end. Is this expected (maybe as part of "simplified subsumtion" in https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#compiler-changes)? If so, how can the code be fixed? If not, is it a compiler bug? (This problem came up while using gi-gtk-3.0.38, where implicit params are used a lot). Josh /home/jchia/gh/ip-bug/src/Lib.hs:10:28: error: • Couldn't match type ‘(?self::Int) => Char’ with ‘Char’ Expected: Int -> Char -> Char Actual: Int -> ((?self::Int) => Char) -> Char • In the expression: foo In the expression: (let ?self = x in foo) :: Int -> Char -> Char In the expression: ((let ?self = x in foo) :: Int -> Char -> Char) x | 10 | bar x = ((let ?self = x in foo) :: Int -> Char -> Char) x | ^^^ /home/jchia/gh/ip-bug/src/Lib.hs:13:26: error: • Couldn't match type ‘(?self::Int) => Char’ with ‘Char’ Expected: Char -> Char Actual: ((?self::Int) => Char) -> Char • In the expression: foo x In the expression: let ?self = x in foo x In an equation for ‘baz’: baz x = let ?self = x in foo x | 13 | baz x = let ?self = x in foo x | ^^^^^