In S_mro_get_linear_isa_c3() replace sv_inc() with code for the cases we need.

1 files changed, 9 insertions, 3 deletions

diff --git a/ext/mro/mro.xs b/ext/mro/mro.xs
index 69557a67d7..00c960dd22 100644
--- a/ext/mro/mro.xs
+++ b/ext/mro/mro.xs
@@ -158,9 +158,15 @@ S_mro_get_linear_isa_c3(pTHX_ HV* stash, U32 level)
                     HE* const he = hv_fetch_ent(tails, seqitem, 1, 0);
                     if(he) {
                         SV* const val = HeVAL(he);
-			/* This will increment undef to 1, which is what we
-			   want for a newly created entry.  */
-                        sv_inc(val);
+                        /* For 5.8.0 and later, sv_inc() with increment undef to
+			   an IV of 1, which is what we want for a newly created
+			   entry.  However, for 5.6.x it will become an NV of
+			   1.0, which confuses the SvIVX() checks above.  */
+			if(SvIOK(val)) {
+			    SvIV_set(val, SvIVX(val) + 1);
+			} else {
+			    sv_setiv(val, 1);
+			}
                     }
                 }
             }