#10347: Spurious "unused constructor" warning with Coercible -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect | Test Case: warning at compile-time | typecheck/should_compile/T10347 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I wonder if you get an unused-import warning if you say {{{ import Foo( N( N ) ) where test :: Test test = Test $ coerce (0::Int) }}} That is, does the problem apply to imported newtype constructors too? I took at quick look at what to do. The obvious thing to do is * Make `tcg_dus` into a `TcRef` (like `tcg_keep` and `tcg_used_gres`) * Make the constraint solve add a use to `tcg_dus` when solving `Coercible` constraints. (We'd need to change the representation of `tcg_dus` from a list to an `OrdList` or something, else we'd bet inefficient appends.) * Move `reportUnusedNames` after the type checker. Standing back a bit, we carefully gather `tcg_dus :: [DefUse]`, were `type DefUse = (Maybe Defs, Uses)`. So from {{{ type S = Int type T = S -> R }}} we the `DefUse` pairs `(Just S, {})`, and `(Just T, {S,R})`. The idea is that if `T` is unused we can report `S` as unused too. A simpler alternative is simply to gather all the binders `{S,T}` and all the occurrences `{S,R}` and report things that are defined but not referred to. That would report `T` but not `S`. If you deleted `T` you'd get a new warning about `S`. But it's debatable whether reporting `S` unused is a feature or a bug. It certainly confusing -- you can see a reference to `S` -- and if the lack of a reference to `T` was a mistake, then the unused-S warning is simply distracting. So my thought is this: * Suppose instead of gathering `[DefUse]` we simply gather a `NameSet` of things that are referred to; and report as unused anything that it not referred to. That would be significantly simpler. The defs are the declarations themselves, of course. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10347#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler