"opaque" field in HH\TypeStructure<T>

[lexidor]

Hi all,

I have been using HH\ReifiedGenerics\get_type_structure<T>() recently to feed some type driven codegen. The HH\TypeStructure<T> shape sometimes contains a "opaque" key with the value true.

hphpd> newtype A = int;
newtype A = int;
hphpd> =\HH\ReifiedGenerics\get_type_structure<A>();
=\HH\ReifiedGenerics\get_type_structure<A>();
Dict
(
    [kind] => 1
    [opaque] => true
    [alias] => "A"
)

But sometimes it goes missing.

hphpd> newtype B = \stdClass;
newtype B = \stdClass;
hphpd> =\HH\ReifiedGenerics\get_type_structure<B>();
=\HH\ReifiedGenerics\get_type_structure<B>();
Dict
(
    [kind] => 15
    [classname] => "stdClass"
    [alias] => "B"
)

I would have expected "opaque" to be present here, but it is not. Going via new ReflectionTypeAlias('B') does yield an "opaque" key, but it seems to use a different "kind" under the hood, namely \HH\TypeStructureKind::OF_UNRESOLVED.

hphpd> = (new \ReflectionTypeAlias('B'))->getTypeStructure();
= (new \ReflectionTypeAlias('B'))->getTypeStructure();
Dict
(
    [kind] => 101
    [classname] => "stdClass"
    [opaque] => true
)

I'd love to know the what and how of this, but I didn't know who to ask.

Also, I tried to figure out whether this was a feature or a bug. This test and expect seems to lean towards the feature side of things.

I do know that this is an experimental feature of hhvm that may need to be removed in the future. I am not using this in ways that couldn't be achieved via other means. This is however a lot simpler than the alternatives. I am aware that I might need to go the alternative route later if / when this function gets removed from hhvm.