diff --git a/src/datatype.c b/src/datatype.c
index c6544eca16606..73b194468de37 100644
--- a/src/datatype.c
+++ b/src/datatype.c
@@ -98,6 +98,8 @@ 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;
 }
 
@@ -554,7 +556,6 @@ 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;
diff --git a/src/dump.c b/src/dump.c
index 7d83ccac3610c..56c8e6129c16e 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -1179,7 +1179,6 @@ 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;
diff --git a/src/jltypes.c b/src/jltypes.c
index ab84096f7b917..94d921cc8de0e 100644
--- a/src/jltypes.c
+++ b/src/jltypes.c
@@ -1189,7 +1189,7 @@ static void check_datatype_parameters(jl_typename_t *tn, jl_value_t **params, si
     JL_GC_POP();
 }
 
-static jl_value_t *extract_wrapper(jl_value_t *t JL_PROPAGATES_ROOT)
+static jl_value_t *extract_wrapper(jl_value_t *t JL_PROPAGATES_ROOT) JL_GLOBALLY_ROOTED
 {
     t = jl_unwrap_unionall(t);
     if (jl_is_datatype(t))
@@ -1255,12 +1255,9 @@ static jl_value_t *inst_datatype_inner(jl_datatype_t *dt, jl_svec_t *p, jl_value
             jl_value_t *tw = extract_wrapper(pi);
             if (tw && tw != pi && (tn != jl_type_typename || jl_typeof(pi) == jl_typeof(tw)) &&
                     jl_types_equal(pi, tw)) {
-                if (jl_is_vararg_type(iparams[i])) {
-                    tw = jl_wrap_vararg(tw, jl_tparam1(jl_unwrap_unionall(iparams[i])));
-                    JL_GC_PUSH1(&tw);
-                    tw = jl_rewrap_unionall(tw, iparams[i]);
-                    JL_GC_POP();
-                }
+                // This would require some special handling, but is never used at
+                // the moment.
+                assert(!jl_is_vararg_type(iparams[i]));
                 iparams[i] = tw;
                 if (p) jl_gc_wb(p, tw);
             }
@@ -1419,7 +1416,6 @@ 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)
@@ -1915,7 +1911,6 @@ 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;
@@ -1934,7 +1929,6 @@ 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;
@@ -1954,7 +1948,6 @@ 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 = 5;
@@ -1967,7 +1960,6 @@ 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;
@@ -1981,7 +1973,6 @@ 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;