Failable init fails to compile in protocol

This protocol requirement:

protocol TestFailableInit {
    init?(_ aSomeValue: String)
}

produces these compile time errors:

Error (E1) opening parenthesis expected, got &
Error (E375) One of “var”, “func”, “subscript”, “typealias”, “__event”, “prefix”, “postfix”, “infix”, “init”, “associatedtype”, expected, got &
Error (E1) closing bracket expected, got &

Regular init compiles fine.

Thanks, logged as bugs://82501