relates to:

• SR-128 Pass array to variadic function
• SR-6386 Note incorrect pass-through of variadic arguments

Issue Description:

Consider this:

import Foundation
import os.log

class OSLogWrapper {

    func logDefault(_ message: StaticString, _ args: CVarArg...) {
        os_log(message, type: .default, args)
    }

    func testWrapper() {
        logDefault("WTF: %f", 1.2345)
    }
}

let logger = OSLogWrapper()
logger.testWrapper()

This code compiles and runs without warning, but produces unexpected output:

2018-06-20 02:50:21.137119-0500 test[39525:6101799] WTF: 0.000000

The bug (in the example code, not in Swift) is that logDefault receives an Array<CVarArg> in its args parameter, and passes it to os_log, which is expecting variadic CVarArg arguments. Since Array conforms to CVarArg, Swift casts args to CVarArg implicitly.

The bug in Swift is that the conversion of Array<CVarArg> to CVarArg is rarely the desired behavior. (What's wanted is argument splatting, but I think that's unlikely to happen any time soon.) Swift should emit a warning when Array<CVarArg> is implicitly converted to CVarArg, and require an explicit cast to silence the warning.

P.S. This issue is based on https://stackoverflow.com/q/50937765/77567