Constraint solver panic due to derived type function equality

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
{-# OPTIONS_GHC -ddump-tc-trace #-}

type family F (x :: *) :: *

class (y ~ F x) => C x y

z = () :: C x y => ()

causes a panic while tracing the typechecker:

solveNestedImplications starting {
  original inerts =  Equalities:
                     Type-function equalities: [D] _ :: F x_aeH ~ y_aeI (CFunEqCan)
                     Dictionaries: [W] $dC_aeK :: C x_aeH y_aeI (CDictCan)
                     Irreds:
                     Insolubles = {}
                     Solved dicts 0
                     Solved funeqs 0
  thinner_inerts  =  Equalities:ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.7.20130812 for i386-unknown-linux):
	No match in record selector ctev_evar

If -ddump-tc-trace is not used, the correct constraint solving error is generated:

    Could not deduce (C x0 y0)
      arising from the ambiguity check for an expression type signature
    from the context (C x y)
      bound by an expression type signature: C x y => ()

It seems that prepareInertsForImplications in TcSMonad assumes that all the inert_funeqs are givens or wanteds; if there are some deriveds, then the above panic results.

Trac metadata

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