codegen: avoid attempting to box singleton objects unnecessarily (#41658

)

Fixes #41615

(cherry picked from commit b692c9b)

src/codegen.cpp

@@ -946,11 +946,13 @@ static bool jl_is_pointerfree(jl_value_t* t)
 
 // these queries are usually related, but we split them out here
 // for convenience and clarity (and because it changes the calling convention)
+// n.b. this must include jl_is_datatype_singleton (ghostType) and primitive types
 static bool deserves_stack(jl_value_t* t)
 {
     if (!jl_is_concrete_immutable(t))
         return false;
-    return jl_datatype_isinlinealloc((jl_datatype_t*)t, 0);
+    jl_datatype_t *dt = (jl_datatype_t*)t;
+    return jl_is_datatype_singleton(dt) || jl_datatype_isinlinealloc(dt, 0);
 }
 static bool deserves_argbox(jl_value_t* t)
 {