peculiar behaviour with default instances and type variables

Here is a small module that has perplexing behaviour under GHC 7.6.1.

In its current form, it compiles and behaves correctly.

There are two lines commented out in the module.

Uncomment the first type signature for "default put", comment out the second one, and recompile. You'll get an "ambiguous constraint" error.

Now uncomment the FlexibleInstances pragma. The error changes to "no instance for (Put a0)", and moves to the bottom of the file.

I encountered this problem earlier today, in a module (in the binary package) that had FlexibleInstances and a default signature where the name of the type variable did not match the name of the class's type variable.

In that module, I did not have an instance like the one at the bottom of the file, so the module compiled cleanly and without error.

It was not until I tried to compile the module that depended on it that the error occurred. It took me three hours of poking around at random before I accidentally figured out what was wrong.

It would be very helpful if GHC either accepted default signatures with non-matching type variables (which seems correct to me) or rejected them, but did one or the other consistently. Finding a mysterious type error in a downstream module because of a very hard-to-spot error in an upstream module turned out to be extremely difficult.

{-# LANGUAGE DefaultSignatures, FlexibleContexts, DeriveGeneric #-}
-- {-# LANGUAGE FlexibleInstances #-}

module Whee where

import GHC.Generics

class GPut f where
    gput :: f a -> [()]

class Put a where
    put :: a -> [()]

    -- default put :: (Generic t, GPut (Rep t)) => t -> [()]
    default put :: (Generic a, GPut (Rep a)) => a -> [()]
    put = gput . from

instance GPut U1 where
    gput U1 = []

instance GPut a => GPut (M1 i c a) where
    gput = gput . unM1

data Foo = Foo
         deriving (Generic)

instance Put Foo

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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