a.txt

run it as:

perl6-m -e 'my @a = <a b c>; my role IP { has $!ip }; @a does IP'
1) body(0x7fcb9e470230)->replaced = 0x49402a0
elems 1: (0x1b86810, 0x1b9d8c8, 0x1b9d8e0)
form_space = 0x7fcb9df2f010, to_space = 0x7fcb9e330010
real_data: 0x1b9d8e0 -> 0x18000900000000
elems 2: 0x18000900000000
Unhandled exception: This type does not support elems
   at src/gen/m-CORE.setting:8231  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm:REIFY:56)
 from src/gen/m-CORE.setting:8569  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm:REIFY:126)
 from src/gen/m-CORE.setting:7443  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm::507)
 from src/gen/m-CORE.setting:7398  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm:reify:74)
 from src/gen/m-CORE.setting:7878  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm:gimme:219)
 from src/gen/m-CORE.setting:8281  (/home/froggs/dev/nqp/install/languages/perl6/runtime/CORE.setting.moarvm:sink:58)
 from src/gen/m-main.nqp:47  (/home/froggs/dev/nqp/install/languages/perl6/runtime/perl6.moarvm:MAIN:39)
 from src/gen/m-main.nqp:41  (/home/froggs/dev/nqp/install/languages/perl6/runtime/perl6.moarvm::207)
 from <unknown>:1  (/home/froggs/dev/nqp/install/languages/perl6/runtime/perl6.moarvm::8)
 from <unknown>:1  (/home/froggs/dev/nqp/install/languages/perl6/runtime/perl6.moarvm::9)

replaced.diff

diff --git a/src/6model/reprs/P6opaque.c b/src/6model/reprs/P6opaque.c
index b72658d..217902f 100644
--- a/src/6model/reprs/P6opaque.c
+++ b/src/6model/reprs/P6opaque.c
@@ -10,10 +10,14 @@ static const MVMREPROps this_repr;
  * uninitialized slot that should trigger auto-viv. */
 static MVMObject *ass_null = NULL;
 
-/* If an object gets mixed in to, we need to be sure we look at is real body,
+/* If an object gets mixed in to, we need to be sure we look at its real body,
  * which may have been moved to hang off the specified pointer. */
 static void * real_data(void *data) {
     MVMP6opaqueBody *body = (MVMP6opaqueBody *)data;
+    if (body->replaced && !((unsigned long long)body->replaced & 0xFFFFFFFF)) {
+        //~ body->replaced = NULL;
+        printf("real_data: %p -> %p\n", data, body->replaced ? body->replaced : data);
+    }
     return body->replaced ? body->replaced : data;
 }
 
@@ -1128,10 +1132,12 @@ void change_type(MVMThreadContext *tc, MVMObject *obj, MVMObject *new_type) {
 
         /* Pointer switch, taking care of existing body issues. */
         if (body->replaced) {
+            printf("1) body(%p)->replaced = %p\n", body, new);
             body->replaced = new;
             free(old);
         }
         else {
+            printf("1) body(%p)->replaced = %p\n", body, new);
             body->replaced = new;
         }
     }
@@ -1270,7 +1276,11 @@ static void delete_key(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, voi
 
 static MVMuint64 elems(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, void *data) {
     MVMP6opaqueREPRData *repr_data = (MVMP6opaqueREPRData *)st->REPR_data;
+    printf("elems 1: (%p, %p, %p)\n", st, root, data);
+    printf("form_space = %p, to_space = %p\n", tc->nursery_fromspace, tc->nursery_tospace);
+    //~ printf("replaced: %p\n", ((MVMP6opaqueBody *)data)->replaced);
     data = real_data(data);
+    printf("elems 2: %p\n", data);
     if (repr_data->pos_del_slot >= 0) {
         MVMObject *del = get_obj_at_offset(data, repr_data->attribute_offsets[repr_data->pos_del_slot]);
         return REPR(del)->elems(tc, STABLE(del), del, OBJECT_BODY(del));
diff --git a/src/6model/reprs/P6opaque.h b/src/6model/reprs/P6opaque.h
index e74b9e2..6ef5278 100644
--- a/src/6model/reprs/P6opaque.h
+++ b/src/6model/reprs/P6opaque.h
@@ -99,3 +99,7 @@ struct MVMP6opaqueREPRData {
 
 /* Function for REPR setup. */
 const MVMREPROps * MVMP6opaque_initialize(MVMThreadContext *tc);
+
+/* If an object gets mixed in to, we need to be sure we look at its real body,
+ * which may have been moved to hang off the specified pointer. */
+MVM_PUBLIC void * MVMP6opaque_real_data(MVMThreadContext *tc, void *data);