Compiler panic with ill-typed code using implicit parameters

The following snippet produces a compiler panic with GHC 7.7.20121221:

{-# LANGUAGE ImplicitParams #-}

hmm :: ()
hmm = let ?a = True in ?a

main = print hmm

Which results in the error message:

[1 of 1] Compiling Main             ( bug.hs, bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20121221 for x86_64-unknown-linux):
        ASSERT failed!
    file compiler/typecheck/TcMType.lhs line 407
    Double update of meta tyvar
      t_ajp{tv} [tau[0]]
      Indirect ghc-prim:GHC.Types.Bool{(w) tc 3c}

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Note that the same snippet compiled with GHC 7.6.1 doesn't panic, and instead gives a type error:

[1 of 1] Compiling Main             ( bug.hs, bug.o )

bug.hs:4:24:
    Couldn't match type `Bool' with `()'
    When using functional dependencies to combine
      ?a::Bool,
        arising from the implicit-parameter bindings for ?a
        at bug.hs:4:7-25
      ?a::(),
        arising from a use of implicit parameter `?a' at bug.hs:4:24-25
    In the expression: ?a
    In the expression: let ?a = True in ?a

Trac metadata

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