Confusing designated initializer rules

[AdamCmiel]

Description

Pretty sure we're breaking rule #1 for initializer delegation here: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/initialization/#Initializer-Delegation-for-Class-Types

If a derived class has a let property that's initialized before calling it's super's convenience initializer, that call is allowed. If the let property is rewritten as a var or removed, the appropriate super init delegation to a designated initializer diagnostic is emitted.

Reproduction

class Base {
    public init(name: String) {
        print(name)
    }

    convenience init(number: Int) {
        self.init(name: "\(number)")
    }
}

class DerivedLetPasses: Base {
    let string: String
    init() {
        self.string = "foo"
        super.init(number: 3) // compilation succeeds
    }
}

class DerivedVarFails: Base {
    var string: String
    init() {
        self.string = "foo"
        super.init(number: 3) // error: Must call a designated initializer of the superclass 'Base'
    }
}

class DerivedNoExtraProperties: Base {
    init() {
        super.init(number: 3) // error: Must call a designated initializer of the superclass 'Base'
    }
}

Expected behavior

I believe that diagnostic should be emitted for all three derived classes, this would be consistent with the docs.

Environment

Xcode version 16.2 (16C5032a)

Additional information

No response

[AdamCmiel]

Possibly a dupe of #65316