reflect: StructOf doesn't support recursive structs

[ben-clayton]

go version: 1.8

I've been playing with StructOf to build go values that match a serialized schema-driven form.
I've encountered an edge case that I'm uncertain whether Go currently supports. Given the struct:

type S struct {
   Ptr *S
}

I see no way to forward-declare the S struct to use it as a pointer field to StructOf.

Is there some way I can build this struct type?

[bradfitz]

/cc @crawshaw

[ianlancetaylor]

You can't do it. This is essentially a dup of #16522. Since you can't build a named type with reflect, you can't build a self-referential type.

[jimmyfrasche]

@ianlancetaylor I don't see how this is essentially a dupe, though it is certainly related (and blocked on) #16522.

With #16522 you could do

s := reflect.StructOf(someFields) //want this to contain n but n doesn't exist yet
n := reflect.NamedOf(s, "S", nil)

but without a mechanism for forward declaration, as mentioned in the OP, how do you add a field to s of type n when the construction of n requires that s was already constructed?

(A related concern came up in #4146 where you would need n to define the signatures of the methods to add to n, but that's easier to handle).

[dockercore]

s := reflect.StructOf(someFields) //want this to contain n but n doesn't exist yet
n := reflect.NamedOf(s, "S", nil)

[cosmos72]

If accepted and implemented, proposal #39528 would solve this