#15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric -------------------------------------+------------------------------------- Reporter: konn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: Resolution: | Keywords: deriving 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 konn):
You're explicitly requesting that you reuse the Generic instance for Bad Foo, and lo and behold, GHC gives you it. I can't imagine the situation where `Generic` should give the different information than its true structure.
Although the code above is simple enough, I think it might be error-prone to allow writing Generic in DerivingVia-clause. As for GND, deriving strategy `newtype` is used as a prefix for the deriving clause, so one can easily notice the mistake; but as for DerivingVia, since `via`-clause is placed after constraints, one can easily overlook that the current deriving strategy.
this code will give an error if combined with the Safe extension
Hence, I think this behaviour should be enabled by default, even without `Safe` extension. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15434#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler