8.0.2 derives invalid code when class method is constrained by itself

GHC 8.0.1 is able to compile this without a problem and doesn't require FlexibleContexts.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ConstrainedClassMethods #-}

module A where

class C m where
  foo :: C m => m ()

newtype N m a = N (m a)
  deriving C

Compare the output of 8.0.1, 8.0.2 and 8.1. I turned on -fdefer-type-errors in order for -ddump-deriv to work.

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.1

$ ghc -ddump-deriv A.hs
[1 of 1] Compiling A                ( A.hs, A.o )

==================== Derived instances ====================
Derived instances:
  instance A.C m_aNK => A.C (A.N m_aNK) where
    A.foo = GHC.Prim.coerce (A.foo :: m_ap1 ()) :: A.N m_ap1 ()


GHC.Generics representation types:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2

$ ghc A.hs
[1 of 1] Compiling A                ( A.hs, A.o )

A.hs:10:12: error:
    • Couldn't match type ‘m’ with ‘N m’
        arising from the coercion of the method ‘foo’
          from type ‘C m => m ()’ to type ‘C (N m) => N m ()’
      ‘m’ is a rigid type variable bound by
        the deriving clause for ‘C (N m)’ at A.hs:10:12
    • When deriving the instance for (C (N m))

$ ghc -ddump-deriv -fdefer-type-errors A.hs
[1 of 1] Compiling A                ( A.hs, A.o )

==================== Derived instances ====================
Derived instances:
  instance A.C m_awm => A.C (A.N m_awm) where
    A.foo
      = GHC.Prim.coerce
          @(A.C m_ap0 => m_ap0 ()) @(A.C (A.N m_ap0) => A.N m_ap0 ()) A.foo


GHC.Generics representation types:



A.hs:11:12: warning: [-Wdeferred-type-errors]
    • Couldn't match type ‘m’ with ‘N m’
        arising from a use of ‘GHC.Prim.coerce’
      ‘m’ is a rigid type variable bound by
        the instance declaration at A.hs:11:12
    • In the expression:
        GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo
      In an equation for ‘foo’:
          foo = GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo
      When typechecking the code for ‘foo’
        in a derived instance for ‘C (N m)’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘C (N m)’
    • Relevant bindings include foo :: N m () (bound at A.hs:11:12)

There's no '8.0.2' version to report this against so I've chosen 8.1. GHC 8.1 gives very similar results:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.1.20160930

$ ghc A.hs
[1 of 1] Compiling A                ( A.hs, A.o )

A.hs:11:12: error:
    • Couldn't match type ‘m’ with ‘N m’
        arising from the coercion of the method ‘foo’
          from type ‘C m => m ()’ to type ‘C (N m) => N m ()’
      ‘m’ is a rigid type variable bound by
        the deriving clause for ‘C (N m)’ at A.hs:11:12
    • When deriving the instance for (C (N m))

$ ghc -ddump-deriv -fdefer-type-errors A.hs
[1 of 1] Compiling A                ( A.hs, A.o )

==================== Derived instances ====================
Derived instances:
  instance A.C m_awK => A.C (A.N m_awK) where
    A.foo
      = GHC.Prim.coerce
          @(A.C m_app => m_app ()) @(A.C (A.N m_app) => A.N m_app ()) A.foo


GHC.Generics representation types:



A.hs:11:12: warning: [-Wsimplifiable-class-constraints]
    The constraint ‘C (N m)’ matches an instance declaration
    instance C m => C (N m) -- Defined at A.hs:11:12
    This makes type inference for inner bindings fragile;
      either use MonoLocalBinds, or simplify it using the instance

A.hs:11:12: warning: [-Wdeferred-type-errors]
    • Couldn't match type ‘m’ with ‘N m’
        arising from a use of ‘GHC.Prim.coerce’
      ‘m’ is a rigid type variable bound by
        the instance declaration at A.hs:11:12
    • In the expression:
        GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo
      In an equation for ‘foo’:
          foo = GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo
      When typechecking the code for ‘foo’
        in a derived instance for ‘C (N m)’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘C (N m)’
    • Relevant bindings include foo :: N m () (bound at A.hs:11:12)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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