Implementation of nullable:: does not work correctly

[popematt]

The Ion Schema Grammar allows for nullable:: to be applied to any type reference.

<TYPE_REFERENCE> ::=           <TYPE_NAME>
                   | nullable::<TYPE_NAME>
                   |           <TYPE_ALIAS>
                   | nullable::<TYPE_ALIAS>
                   |           <TYPE_DEFINITION>
                   | nullable::<TYPE_DEFINITION>
                   |           <IMPORT_TYPE>
                   | nullable::<IMPORT_TYPE>

And the example in the Core Types section of the spec says:

The core types do not include any of Ion's null.* values, but each of the types may have a weakly- or strongly-typed null value if the type name is annotated with nullable. When a strongly-typed null value is encountered, its type must agree with one of the core types of the expected type. For example, if a any_of: nullable::[int, string, struct] is expected, 5, "hi", {}, null, null.null, null.int, null.string, and null.struct are all valid values, but null.decimal is not.

Even ignoring the fact that the example does not match the grammar (nullable:: should be on a type reference, not a list of type references), the ion-schema-kotlin implementation does not do what this says. I also tried several other type definitions that are similar to the example, and the Ion Schema System consistently failed to allow the null values or ran into a StackOverflowError, unless I specifically used an ion type instead of a core type.

Given the test cases mentioned in the quoted paragraph, here are the results for the type in the example, and several similar types.

valid::[ 5, "hi", {}, null, null.null, null.int, null.string, null.struct ]
invalid::[ null.decimal ]

Type	Result
type::{any_of:nullable::[int, string, struct]}	test fails for null, null.null, null.struct, null.int, null.string
type::{type:nullable::{any_of:[int, string, struct]}}	stack overflow error for null.decimal, null.struct, null.int, null.string
type::{any_of:[nullable::int, nullable::string, nullable::struct]}	test fails for null, null.null, null.struct, null.int, null.string
type::{type:nullable::any, any_of:[nullable::int, nullable::string, nullable::struct]}	stack overflow error for null.decimal, null.struct, null.int, null.string
type::{type:nullable::any}	stack overflow error for null.decimal, null.struct, null.int, null.string
type::{type:$any, any_of:[nullable::int, nullable::string, nullable::struct]}	All test cases pass

[popematt]

Also created amazon-ion/ion-schema#38 to address the ambiguity in the spec around nullable::.