Why is the return value of String.addingPercentEncoding() optional?

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I filed a bug report with Apple about this, and heard back — with a very helpful response, no less!

Turns out (much to my surprise) that it’s possible to successfully create Swift strings that contain invalid Unicode in the form of unpaired UTF-16 surrogate chars. Such a string can cause UTF-8 encoding to fail. Here’s some code that illustrates this behavior:

// Succeeds (wat?!):
let str = String(
    bytes: [0xD8, 0x00] as [UInt8],
    encoding: .utf16BigEndian)!

// Returns nil:
str.addingPercentEncoding(withAllowedCharacters: .alphanumerics)

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