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Issue Description:

I'm trying to decode a JSON object that has a property whose name is the sole unicode character FEFF, which does not work. 200B works, for example. This is the type:

struct TopLevel: Codable {

{{ let empty: String}}

{{}}}

extension TopLevel {

{{ enum CodingKeys: String, CodingKey {}}

{{ case empty = "\u{feff}"}}

}

{{}}}

and this is the JSON:

{

{{ "\ufeff": "foo"}}

{{}}}

I've attached a test case. Compile it and invoke it with

./a.out blns-object.json