Typeclass membership doesn't bring coercion superclass requirements into scope

-- This works:
class C1 a where c1 :: a -> Int
class C1 a => C2 a where c2 :: a -> Int
test :: C2 a => a -> Int
test a = c1 a + c2 a

-- This doesn't work:
class (a ~ b) => E a b
cast :: E a b => a -> b
cast a = a

{- error:
tyfam.hs:36:9:
    Couldn't match expected type `b' against inferred type `a'
      `b' is a rigid type variable bound by
          the type signature for `cast' at tyfam.hs:35:14
      `a' is a rigid type variable bound by
          the type signature for `cast' at tyfam.hs:35:12
    In the expression: a
    In the definition of `cast': cast a = a
 -}

I would expect that the requirement of membership in E a b would bring (a ~ b) into scope and allow cast to typecheck.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC
Operating system	Unknown
Architecture	Unknown