#15052: DeriveAnyClass instances may skip TypeError constraints -------------------------------------+------------------------------------- Reporter: jcpetruzza | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: | CustomTypeErrors 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 jcpetruzza): I'm not sure I see why this would be a breaking change. In this last example (which is the important one, the original was an oversimplification, we can disregard it), why would these two be any different: {{{#!haskell data TS = TSL | TSR deriving (Gen.Generic, C) }}} vs {{{#!haskell data TS = TSL | TSR deriving (Gen.Generic) instance C TS }}} The latter fails, IIUC, because we are trying to see if the default signature of `f` in `C` applies to `TS`, and this forces the reduction of `GC (Rep TS)`, that eventually leads to the `TypeError`. This is the standard behaviour. Am I mistaken in thinking that these two declarations should be equivalent? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15052#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler