relates to:

• SR-11095 Warn on pointless default argument

Issue Description:

The Swift compiler accepts the following code:

struct S {
    let i: Int
    init(_ i: Int = 5) {
        self.i = i
    }
    
    init() {
        i = 6
    }
}
print(S().i) // prints 6

This is dangerous, because if S.init() were deleted, the program would still compile; however, S().i would now evaluate to 5.

Conversely, if init() didn't already exist, and was added later, especially in another module as a convenience init, there might be difficult to debug issues in modules it was exposed to, as which init was called might be inconsistent depending on the location of the new init() function and which modules depended on it (I haven't checked this, but it seems likely).

For these reasons, I believe that this program should be rejected by the compiler.