fix a bug where jl_typeof(jl_emptytuple) was a singleton type missing…

… from the serializer known objects list

this also serializes the datatype along with any singleton.
this simplifies the back-and-forth a bit,
but more importantly, during incremental compilation, it was possible to
encounter a singleton without ever encountering its datatype.
it is now trivially possible to remove the Singleton_tag

src/dump.c

@@ -794,7 +794,14 @@ static void jl_serialize_value_(ios_t *s, jl_value_t *v)
  else {
  if (v == t->instance) {
  assert(mode != MODE_MODULE_POSTWORK);
+ if (mode == MODE_MODULE) {
+ // also flag this in the backref table as special
+ uptrint_t *bp = (uptrint_t*)ptrhash_bp(&backref_table, v);
+ assert(*bp != (uptrint_t)HT_NOTFOUND);
+ *bp |= 1;
+ }
  writetag(s, (jl_value_t*)Singleton_tag);
+ jl_serialize_value(s, t);
  return;
  }
  if (t->size <= 255) {
@@ -983,7 +990,6 @@ static jl_value_t *jl_deserialize_datatype(ios_t *s, int pos, jl_value_t **loc)
  if (has_instance) {
  assert(mode != MODE_MODULE_POSTWORK); // there shouldn't be an instance on a type with uid = 0
  dt->instance = jl_deserialize_value(s, &dt->instance);
- jl_set_typeof(dt->instance, dt);
  jl_gc_wb(dt, dt->instance);
  }
  if (tag == 5) {
@@ -1382,7 +1388,13 @@ static jl_value_t *jl_deserialize_value_(ios_t *s, jl_value_t *vtag, jl_value_t
  return v;
  }
  else if (vtag == (jl_value_t*)Singleton_tag) {
- assert(mode != MODE_MODULE_POSTWORK);
+ if (mode == MODE_MODULE_POSTWORK) {
+ uptrint_t pos = backref_list.len;
+ arraylist_push(&backref_list, NULL);
+ jl_datatype_t *dt = (jl_datatype_t*)jl_deserialize_value(s, NULL);
+ backref_list.items[pos] = dt->instance;
+ return dt->instance;
+ }
  jl_value_t *v = (jl_value_t*)jl_gc_alloc_0w();
  if (usetable) {
  uptrint_t pos = backref_list.len;
@@ -1393,6 +1405,8 @@ static jl_value_t *jl_deserialize_value_(ios_t *s, jl_value_t *vtag, jl_value_t
  arraylist_push(&flagref_list, (void*)pos);
  }
  }
+ jl_datatype_t *dt = (jl_datatype_t*)jl_deserialize_value(s, NULL); // no loc, since if dt is replaced, then dt->instance would be also
+ jl_set_typeof(v, dt);
  return v;
  }
  assert(0);
@@ -1926,13 +1940,14 @@ static jl_array_t *_jl_restore_incremental(ios_t *f)
  while (j < flagref_list.len) {
  jl_value_t **loc = (jl_value_t**)flagref_list.items[j];
  int offs = (int)(intptr_t)flagref_list.items[j+1];
- if (*loc == (jl_value_t*)dt) {
+ jl_value_t *o = loc ? *loc : (jl_value_t*)backref_list.items[offs];
+ if ((jl_value_t*)dt == o) {
  if (t != dt) {
  if (loc) *loc = (jl_value_t*)t;
  if (offs > 0) backref_list.items[offs] = t;
  }
  }
- else if (*loc == v) {
+ else if (v == o) {
  if (t->instance != v) {
  *loc = t->instance;
  if (offs > 0) backref_list.items[offs] = t->instance;
@@ -2037,6 +2052,7 @@ void jl_init_serializer(void)
  (void*)Int32_tag, (void*)Array1d_tag, (void*)Singleton_tag,
  jl_module_type, jl_tvar_type, jl_lambda_info_type,
  (void*)CommonSym_tag, (void*)NearbyGlobal_tag, jl_globalref_type,
+ // everything above here represents a class of object rather only than a literal
 
  jl_emptysvec, jl_emptytuple, jl_false, jl_true, jl_nothing, jl_any_type,
  call_sym, goto_ifnot_sym, return_sym, body_sym, line_sym,
@@ -2093,7 +2109,7 @@ void jl_init_serializer(void)
  jl_ANY_flag, jl_array_any_type, jl_intrinsic_type, jl_method_type,
  jl_methtable_type, jl_voidpointer_type, jl_newvarnode_type,
  jl_array_symbol_type, jl_anytuple_type, jl_tparam0(jl_anytuple_type),
-
+ jl_typeof(jl_emptytuple),
  jl_symbol_type->name, jl_gensym_type->name, jl_tuple_typename,
  jl_ref_type->name, jl_pointer_type->name, jl_simplevector_type->name,
  jl_datatype_type->name, jl_uniontype_type->name, jl_array_type->name,