Extended defaulting rules not used for super classes

Consider the following module

{-# LANGUAGE ExtendedDefaultRules #-}
module Bug4 where

class (Num a) => C a where
    f :: a -> a

instance C Integer where
    f = (+1)

x = show (f 5)

The type of x is

x :: (C a) => String

and since a is ambiguous the defaulting rules should be used. Since Num is a superclass of C the type variable should be defaulted to Integer. But instead we get an error.

Removing the Num superclass of C makes it work, since the type of x is

x ::: (Num a, C a) => String

and the defaulting kicks in as expected.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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