Some(U) of an @as(undefined) case compiles to undefined when U is a constant #8954

Some(U), where U is an @as(undefined) case of an untagged variant, compiles to undefined when U is a constant argument, so it equals None.

@unboxed type u = | @as(undefined) U | V(string)
let x: option<u> = Some(U)                       // compiles to: let x = undefined;
@inline(never) let mk = (v: u): option<u> => Some(v)
Console.log2(x == None, mk(U) == None)

Output: true false. Through a function, Some(U) is boxed and differs from None; as a constant, it is undefined.

Expected: Some(U) differs from None in both cases. This is the example of #6222, which made option unboxing compatible with untagged variants.

A likely place is compiler/ml/lambda.ml, const_some_none_depth, which treats every Const_constructor as not none, including one whose literal is undefined. This cause is not confirmed. The rule is compiler/ml/UntaggedVariants.md rule 12, in #8952.