I'm not sure, but I'd expect this to be undefined behavior because the empty array is not a valid C string since the terminating null byte is missing. In contrast, this works fine:

String(decodingCString: "", as: UTF8.self) // returns ""

because the compiler adds the null byte. But it doesn‘t do that for an array. A properly null-terminated C string in array form should look like this:

String(decodingCString: [0], as: UTF8.self) // returns ""

@ole this doesn't look like a bug to me. If you pass a String there "", Swift will do some magic which under the hood will actually call "".withCString for you. withCString will always give you a NUL terminated String. So that's consistent with what you see.

For array, there's also special Swift magic involved and it'll do withUnsafePointer on the array but that might or might not happen to be 0 terminated.

So I'd expect to see exactly what you're seeing. So String(decodingCString: [], as: Unicode.UTF8.self) does trigger undefined behaviour (as you're reading into arbitrary bits of memory until you find a 0) and may return whatever it wants.

I think the behaviour you want is String(decoding: [], as: Unicode.UTF8.self) which is defined on Collection and won't read past the end.

@weissi I agree it's not a bug.