uppercased

Because case conversion can result in multiple characters, the result of uppercased() is a string.

let chars: [Character] = ["e", "é", "и", "π", "ß", "1"]
for ch in chars {
    print(ch, "-->", ch.uppercased())
}
// Prints:
// e --> E
// é --> É
// и --> И
// π --> Π
// ß --> SS
// 1 --> 1