Fix VoidRep handling in ghci debugger

'go' inside extractSubTerms was giving a bad result given a VoidRep,
attempting to round towards the next multiple of 0.
I don't understand much about the debugger but the code should be better
than it was.

Fixes #24306

compiler/GHC/Runtime/Heap/Inspect.hs

@@ -888,13 +888,13 @@ extractSubTerms recurse clos = liftM thdOf3 . go 0 0
            (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
            return (ptr_i, arr_i, unboxedTupleTerm ty terms0 : terms1)
       | otherwise
-      = case typePrimRepArgs ty of
-          rep_ty :| [] ->  do
+      = case typePrimRep ty of
+          [rep_ty] -> do
             (ptr_i, arr_i, term0)  <- go_rep ptr_i arr_i ty rep_ty
             (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
             return (ptr_i, arr_i, term0 : terms1)
-          rep_ty :| rep_tys -> do
-           (ptr_i, arr_i, terms0) <- go_unary_types ptr_i arr_i (rep_ty:rep_tys)
+          rep_tys -> do
+           (ptr_i, arr_i, terms0) <- go_unary_types ptr_i arr_i rep_tys
            (ptr_i, arr_i, terms1) <- go ptr_i arr_i tys
            return (ptr_i, arr_i, unboxedTupleTerm ty terms0 : terms1)
 

testsuite/tests/ghci.debugger/scripts/T24306.hs

+{-# LANGUAGE UnboxedTuples, UnliftedNewtypes, DataKinds, MagicHash #-}
+module T24306 where
+
+import GHC.Exts
+
+newtype A = MkA (# #)
+data T = T Int# A Int#
+
+x = T 1# (MkA (# #)) 2#

testsuite/tests/ghci.debugger/scripts/T24306.script

+:load T24306
+:force x

testsuite/tests/ghci.debugger/scripts/T24306.stdout

+x = T 1 (MkA (##)) 2

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

@@ -140,3 +140,4 @@ test('break030',
   ['break030.script'],
 )
 test('T23057', [only_ghci, extra_hc_opts('-fno-break-points')], ghci_script, ['T23057.script'])
+test('T24306', normal, ghci_script, ['T24306.script'])