Derived types unequal with let binding

The types of x and y are different. This breaks the replacing equals with equals gives equals property. I am not sure whether this is a bug, but I would be very keen to know whether it is.

class X l z b m | l -> z b where 
  p:: (Show a, Show b, Show b') => l a b -> m (l a b')
  e::(Show a, Show b) => l a b ->  m [z]

foo :: (X l a b m, Show b, Show a1) => [a] -> l a1 b -> m [(Int, Int, b)]
foo = undefined
o :: (Show z1, Show b,  X l z b m) =>(Int, Int, b) -> l a1 b -> m (l z1 b)
o = undefined

x f r = do
 es <- e r
 something <- p r
 xxx <- foo es r
 foldM 
  (flip o) 
   something
   (map f xxx)

y f r = do
 es <- e r
 something <- p r
 xxx <- foo es r
 let abc = map f xxx
 foldM 
  (flip o) 
   something
   abc

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown