ghc-api/T8628 fails assert on debugged ghc

1. /validate --slow fails thusly:

Unexpected failures:
   ghc-api  T8628 [bad stderr] (normal)

# run by hands:
$ LANG=C make fulltest stage=2 TEST=T8628

=====> T8628(normal) 3453 of 4083 [0, 0, 0] 
cd ./ghc-api && $MAKE -s --no-print-directory T8628    </dev/null >T8628.run.stdout 2>T8628.run.stderr
Actual stderr output differs from expected:
--- /dev/null   2014-06-09 18:31:31.993762230 +0300
+++ ./ghc-api/T8628.run.stderr  2014-08-08 16:02:17.000000000 +0300
@@ -0,0 +1,2 @@
+WARNING: file compiler/simplCore/SimplEnv.lhs, line 539 $fShowX
+WARNING: file compiler/simplCore/SimplEnv.lhs, line 539 $fShowX
*** unexpected failure for T8628(normal)

compiler/simplCore/SimplEnv.lhs:539

refine :: InScopeSet -> Var -> Var
refine in_scope v
  | isLocalId v = case lookupInScope in_scope v of
                  Just v' -> v'
{- 539 -}         Nothing -> WARN( True, ppr v ) v  -- This is an error!
  | otherwise = v

Is it really a bug as comment suggests?

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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