Relax 'Cannot use a polymorphic type for a compound literal' restriction to support the use of more flexible wrapper types.

[JesseRMeyer]

I propose that the limitation: Error: Cannot use a polymorphic type for a compound literal, got '$Q'

to be lifted, to allow for constructions like:

ThreadSafeQueue :: struct($Q: typeid) {
    queue: Q,
    futex: os_Futex,

    _T: type_of(Q{}._T[0]), // Currently errors
}

ThreadSafeQueue_Pop :: proc(q: ^$Q) -> type_of(Q{}._T[0]) { // return type errors
    Lock(&q.futex)
    defer Unlock(&q.futex)

    ensure(!IsEmpty(&q.queue))
    return Pop(&q.queue)
}

Where $Q may resemble (in addition to supporting procedures like Pop):

Queue :: struct($T: typeid) {
  .. ordinary queue fields ..,
  _T: [0]T, // 0 sized field that stores the type T the queue operates on.  Emulates `using value_type = T;` from C++
}

Lifting this restriction allows for relatively seamless wrapper types that leverage preexisting duck-typing parapoly functionality. Otherwise this is simply a hard restriction on the composition capabilities of Odin. As it stands, the wrapping type ThreadSafeQueue has no way to know what type to provide as a return type (or an an input parameter for Put). Type erasure is an option but it shouldn't be the only one.

My hope is that the error exists as a preventative measure that can be relaxed as opposed to a design goal.

[JesseRMeyer]

gB has designated this limitation as a bug on Discord.