#16322: "deriving newtype instance" generates an infinite loop -------------------------------------+------------------------------------- Reporter: paf31 | Owner: (none) Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by paf31): I'll break down my understanding step-by-step based on what you've said above, and please let me know where I go wrong: 1. I require `C (X String a) (X String b)` given `C a b` 2. `deriving newtype` uses the newtype only in the last type argument, so GHC reduces this to looking for an instance of `C (X String a) b`. 3. The only instances that could ever possibly be applicable (even just based on the class) are `C Int Int`, `C a b` (in scope locally) and `C (X String a) (X String b)` itself (the recursive instance which is apparently chosen) but each leads to a contradiction (note that the functional dependency means I can infer the first type argument from the second): - `C Int Int` fails via `Int ~ X String a`. - `C a b` forces `X String a ~ a` - `C (X String a) (X String b)` forces `X String b ~ b`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16322#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler