[GHC] #10722: GHC accepts "instance forall b. Blah []" (was: GHC generates wrong unused type argument warning(ScopedTypeVariables related))

[GHC]

#10722: GHC accepts "instance forall b. Blah []"
-------------------------------------+-------------------------------------
        Reporter:  osa1              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by rwbarton):

 * failure:  None/Unknown => GHC accepts invalid program


Old description:

> {{{#!hs
> {-# LANGUAGE KindSignatures, ScopedTypeVariables #-}
>
> {-# OPTIONS_GHC -Wall #-}
>
> module Main where
>
> import Data.Proxy
>
> class Blah (a :: * -> *) where
>   f :: a b -> b
>
> instance forall b . Blah [] where
>   f a = let (_ :: Proxy b) = undefined in head a
>
> main :: IO ()
> main = return ()
> }}}
>
> When I run this using GHC 7.10.1 and GHC HEAD, I'm getting this warnings:
>
> {{{
> [1 of 1] Compiling Main             ( Main.hs, Main.o )
>
> Main.hs:12:17: warning:
>     Unused quantified type variable ‘b’
>     In the type ‘forall b. Blah []’
>     In an instance declaration
>
> Main.hs:13:13: warning:
>     This pattern-binding binds no variables: (_ :: Proxy b) = undefined
> Linking Main ...
> }}}
>
> First warning is wrong. `b` is used, and this program doesn't compile if
> I remove that `b`.

New description:

 {{{#!hs
 {-# LANGUAGE KindSignatures, ScopedTypeVariables #-}

 {-# OPTIONS_GHC -Wall #-}

 module Main where

 import Data.Proxy

 class Blah (a :: * -> *) where
   f :: a b -> b

 instance forall b . Blah [] where
   f a = let (_ :: Proxy b) = undefined in head a

 main :: IO ()
 main = return ()
 }}}

 When I run this using GHC 7.10.1 and GHC HEAD, I'm getting this warnings:

 {{{
 [1 of 1] Compiling Main             ( Main.hs, Main.o )

 Main.hs:12:17: warning:
     Unused quantified type variable ‘b’
     In the type ‘forall b. Blah []’
     In an instance declaration

 Main.hs:13:13: warning:
     This pattern-binding binds no variables: (_ :: Proxy b) = undefined
 Linking Main ...
 }}}

 but the program compiles successfully.

--

Comment:

 Assuming you wanted to refer to the type variable `b` in the type of `f`,
 you should use InstanceSigs as described in section 7.6.3.6 of
 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/type-
 class-extensions.html:

 {{{
 instance Blah [] where
   f :: forall b. [b] -> b
   f a = let (_ :: Proxy b) = undefined in head a
 }}}

 `instance forall b . Blah []` is meaningless, and should be rejected. I
 edited the ticket description accordingly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10722#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler