ensure that instances are only created for types with unique ids

fix #16301
doesn't completely fix #12096, but avoids the symptoms

src/interpreter.c

@@ -384,7 +384,7 @@ static jl_value_t *eval(jl_value_t *e, jl_value_t **locals, jl_lambda_info_t *la
             jl_rethrow();
         }
         jl_compute_field_offsets(dt);
-        if (para == (jl_value_t*)jl_emptysvec && jl_is_datatype_singleton(dt)) {
+        if (para == (jl_value_t*)jl_emptysvec && jl_is_datatype_make_singleton(dt)) {
             dt->instance = newstruct(dt);
             jl_gc_wb(dt, dt->instance);
         }

src/jltypes.c

@@ -2183,7 +2183,7 @@ static jl_value_t *inst_datatype(jl_datatype_t *dt, jl_svec_t *p, jl_value_t **i
     jl_precompute_memoized_dt(ndt);
 
     // assign uid as early as possible
-    if (cacheable && !ndt->abstract && ndt->uid==0)
+    if (cacheable && !ndt->abstract)
         ndt->uid = jl_assign_type_uid();
 
     if (istuple)
@@ -2207,7 +2207,7 @@ static jl_value_t *inst_datatype(jl_datatype_t *dt, jl_svec_t *p, jl_value_t **i
             else {
                 jl_compute_field_offsets(ndt);
             }
-            if (jl_is_datatype_singleton(ndt)) {
+            if (jl_is_datatype_make_singleton(ndt)) {
                 ndt->instance = newstruct(ndt);
                 jl_gc_wb(ndt, ndt->instance);
             }
@@ -3416,7 +3416,8 @@ void jl_init_types(void)
 
     jl_tupletype_t *empty_tuple_type = jl_apply_tuple_type(jl_emptysvec);
     empty_tuple_type->uid = jl_assign_type_uid();
-    jl_emptytuple = ((jl_datatype_t*)empty_tuple_type)->instance;
+    jl_emptytuple = newstruct(empty_tuple_type);
+    empty_tuple_type->instance = jl_emptytuple;
 
     // non-primitive definitions follow
     jl_int32_type = NULL;

src/julia.h

@@ -898,9 +898,13 @@ STATIC_INLINE int jl_isbits(void *t)   // corresponding to isbits() in julia
 
 STATIC_INLINE int jl_is_datatype_singleton(jl_datatype_t *d)
 {
-    return (d->instance != NULL ||
-            (!d->abstract && d->size == 0 && d != jl_sym_type && d->name != jl_array_typename &&
-             (d->name->names == jl_emptysvec || !d->mutabl)));
+    return (d->instance != NULL);
+}
+
+STATIC_INLINE int jl_is_datatype_make_singleton(jl_datatype_t *d)
+{
+    return (!d->abstract && d->size == 0 && d != jl_sym_type && d->name != jl_array_typename &&
+            d->uid != 0 && (d->name->names == jl_emptysvec || !d->mutabl));
 }
 
 STATIC_INLINE int jl_is_abstracttype(void *v)

test/core.jl

@@ -4127,3 +4127,12 @@ let a = Any[]
     @test (push!(a,10); f()) - (push!(a,2); f()) == 8
     @test a == [10, 2]
 end
+
+# issue #12096
+let a = Val{Val{TypeVar(:_, Int, true)}},
+    b = Val{Val{TypeVar(:_, Int)}}
+
+    @test !isdefined(a, :instance)
+    @test  isdefined(b, :instance)
+    @test isleaftype(b)
+end