Unsatisfiable default signature

I discovered the -XDefaultSignatures extension and promptly wrote a default method that I feel should be rejected by the compiler:

{-# LANGUAGE DefaultSignatures #-}

module Foo where

class Foo a where
  foo :: proxy a -> b -> Int
  default foo :: (Integral b) => proxy a -> b -> Int
  foo _ b = fromIntegral b

GHC accepts this code, but rejects the instance instance Foo Bool because No instance for (Integral b) arising from a use of ‘Foo.$gdmfoo’. It seems to me that the class should have never compiled in the first place.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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