Untagged variant puts one typed array in another typed array's case #8955

An untagged variant with cases for two typed arrays accepts a value of one typed array in the case of the other, and the value then matches the other case.

@unboxed type ta = I8(Int8Array.t) | U8(Uint8Array.t)
let f = v => switch v { | I8(_) => "i8" | U8(_) => "u8" }
Console.log(f(I8(Uint8Array.fromArray([1]))))

Output: u8.

The typed-array types are abbreviations of one type, so I8(Uint8Array…) type-checks. The cases are classified by the type name as written, so the run-time check is instanceof Uint8Array.

Expected: either the cases are distinct types, so that I8(Uint8Array…) is a type error, or the variant is rejected because two cases share one type.

The rule is compiler/ml/UntaggedVariants.md rule 5, in #8952. #8950 changes how these types are recognized.