'Untouchable' error with constraint variable in rank-2 type

The following program compiles fine with GHC 7.6.3:

{-# LANGUAGE ConstraintKinds, RankNTypes, GADTs #-}

data Dict c where Dict :: c => Dict c

foo :: Dict c -> (c => r) -> r
foo Dict x = x

bar :: Dict ()
bar = Dict

main :: IO ()
main = print $ foo bar "Hello"

However, it produces the following error with HEAD:

ConstraintIssue.hs:12:8:
    No instance for (Show s0) arising from a use of ‛print’
    The type variable ‛s0’ is ambiguous
    Note: there are several potential instances:
      instance Show Double -- Defined in ‛GHC.Float’
      instance Show Float -- Defined in ‛GHC.Float’
      instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
        -- Defined in ‛GHC.Real’
      ...plus 24 others
    In the expression: print
    In the expression: print $ foo bar "Hello"
    In an equation for ‛main’: main = print $ foo bar "Hello"

ConstraintIssue.hs:12:24:
    Couldn't match expected type ‛s0’ with actual type ‛[Char]’
      ‛s0’ is untouchable
        inside the constraints (())
        bound by a type expected by the context: (()) => s0
        at ConstraintIssue.hs:12:16-30
    In the second argument of ‛foo’, namely ‛"Hello"’
    In the second argument of ‛($)’, namely ‛foo bar "Hello"’
    In the expression: print $ foo bar "Hello"

If the type signature of 'main' is removed, the code compiles OK.

If the empty constraint () is replaced with an equality constraint such as Int ~ Int, then the above error occurs with both 7.6.3 and HEAD, which I guess is intended behaviour.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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