RtClosureInspect: Fix off-by-one error in cvReconstructType

Replaced error-prone index manipulation on a pointer array with
a simple fold on the array elements.

Test Plan: Added a test case that triggers the bug

Reviewers: hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2439

GHC Trac Issues: #12458

compiler/ghci/RtClosureInspect.hs

@@ -898,10 +898,9 @@ cvReconstructType hsc_env max_depth old_ty hval = runTR_maybe hsc_env $ do
         (_,mb_dc)    <- tryTcErrs (tcLookupDataCon dcname)
         case mb_dc of
           Nothing-> do
-                     --  TODO: Check this case
-            forM [0..length (elems $ ptrs clos)] $ \i -> do
-                        tv <- newVar liftedTypeKind
-                        return$ appArr (\e->(tv,e)) (ptrs clos) i
+            forM (elems $ ptrs clos) $ \a -> do
+              tv <- newVar liftedTypeKind
+              return (tv, a)
 
           Just dc -> do
             arg_tys <- getDataConArgTys dc my_ty

testsuite/tests/ghci.debugger/scripts/T12458.script

+data D a = D
+d = D
+:print d
+d `seq` ()

testsuite/tests/ghci.debugger/scripts/T12458.stdout

+d = (_t1::D a)
+()

testsuite/tests/ghci.debugger/scripts/all.T

@@ -86,3 +86,4 @@ test('T2740', normal, ghci_script, ['T2740.script'])
 test('getargs', normal, ghci_script, ['getargs.script'])
 test('T7386', normal, ghci_script, ['T7386.script'])
 test('T8557', normal, ghci_script, ['T8557.script'])
+test('T12458', normal, ghci_script, ['T12458.script'])