Move instance initialization into jl_uninitialized_datatype

Otherwise it's illegal to hold onto this object without having
set ->instance, which is just asking for crashes. Fixes #36649.

(cherry picked from commit d778a3d)

src/datatype.c

@@ -97,6 +97,8 @@ jl_datatype_t *jl_new_uninitialized_datatype(void)
  t->has_concrete_subtype = 1;
  t->layout = NULL;
  t->names = NULL;
+ t->types = NULL;
+ t->instance = NULL;
  return t;
 }
 
@@ -553,7 +555,6 @@ JL_DLLEXPORT jl_datatype_t *jl_new_datatype(
  t->abstract = abstract;
  t->mutabl = mutabl;
  t->ninitialized = ninitialized;
- t->instance = NULL;
  t->size = 0;
 
  t->name = NULL;

src/dump.c

@@ -1463,7 +1463,6 @@ static jl_value_t *jl_deserialize_datatype(jl_serializer_state *s, int pos, jl_v
  assert(pos == backref_list.len - 1 && "nothing should have been deserialized since assigning pos");
  backref_list.items[pos] = dt;
  dt->size = size;
- dt->instance = NULL;
  dt->abstract = flags & 1;
  dt->mutabl = (flags >> 1) & 1;
  int has_layout = (flags >> 2) & 1;

src/jltypes.c

@@ -1309,7 +1309,6 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
  }
  ndt->mutabl = dt->mutabl;
  ndt->abstract = dt->abstract;
- ndt->instance = NULL;
  ndt->size = 0;
  jl_precompute_memoized_dt(ndt, cacheable);
  if (istuple)
@@ -1771,7 +1770,6 @@ void jl_init_types(void) JL_GC_DISABLED
  jl_any_type, jl_any_type, jl_any_type, jl_any_type,
  jl_any_type, jl_any_type, jl_any_type, jl_any_type,
  jl_any_type);
- jl_datatype_type->instance = NULL;
  jl_datatype_type->abstract = 0;
  // NOTE: types are not actually mutable, but we want to ensure they are heap-allocated with stable addresses
  jl_datatype_type->mutabl = 1;
@@ -1790,7 +1788,6 @@ void jl_init_types(void) JL_GC_DISABLED
  jl_typename_type->types = jl_svec(9, jl_symbol_type, jl_any_type, jl_simplevector_type,
  jl_type_type, jl_simplevector_type, jl_simplevector_type,
  jl_any_type, jl_any_type, jl_any_type);
- jl_typename_type->instance = NULL;
  jl_typename_type->abstract = 0;
  jl_typename_type->mutabl = 1;
  jl_typename_type->ninitialized = 2;
@@ -1809,7 +1806,6 @@ void jl_init_types(void) JL_GC_DISABLED
  jl_any_type, jl_any_type/*module*/,
  jl_any_type/*any vector*/, jl_any_type/*long*/, jl_any_type/*int32*/,
  jl_any_type/*uint8*/, jl_any_type/*uint8*/);
- jl_methtable_type->instance = NULL;
  jl_methtable_type->abstract = 0;
  jl_methtable_type->mutabl = 1;
  jl_methtable_type->ninitialized = 4;
@@ -1822,7 +1818,6 @@ void jl_init_types(void) JL_GC_DISABLED
  jl_symbol_type->parameters = jl_emptysvec;
  jl_symbol_type->name->names = jl_emptysvec;
  jl_symbol_type->types = jl_emptysvec;
- jl_symbol_type->instance = NULL;
  jl_symbol_type->size = 0;
  jl_symbol_type->abstract = 0;
  jl_symbol_type->mutabl = 1;
@@ -1836,7 +1831,6 @@ void jl_init_types(void) JL_GC_DISABLED
  jl_simplevector_type->parameters = jl_emptysvec;
  jl_simplevector_type->name->names = jl_emptysvec;
  jl_simplevector_type->types = jl_emptysvec;
- jl_simplevector_type->instance = NULL;
  jl_simplevector_type->abstract = 0;
  jl_simplevector_type->mutabl = 1;
  jl_simplevector_type->ninitialized = 0;