Swift compiler don't detect missing default initializer in generic at compile time.

[barabashd]

Description

The Swift compiler accepts generic <T: UIView> code that calls a default init from that type - T() without verifying that the generic type parameter T has a default initializer still available. This results in a runtime crash instead of a compile-time error, representing a gap in Swift's type system checking.

The issue occurs when a UIView subclass overrides init without providing a default initializer, but the compiler still allows T() instantiation in generic contexts where T: UIView. This creates a silent failure mode where compile-time constraints don't match runtime requirements, undermining Swift's safety guarantees.
This affects common SwiftUI development patterns where developers create reusable generic UIViewRepresentable components, making it a practical rather than theoretical concern.

Reproduction

// call this to reproduce a crash
_ = ViewThatCrash<CustomView>()

struct ViewThatCrash<T: UIView>: UIViewRepresentable {
  func makeUIView(context: Context) -> T {
    T() // crash here
  }
  
  func updateUIView(_ uiView: T, context: Context) {}
}

class CustomView: UIView {
  // changed default init
  init(from str: String) { super.init(frame: .zero) }
  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
}

Stack dump

TestApp/TestView.swift:21: Fatal error: Use of unimplemented initializer 'init(frame:)' for class 'TestApp.TestView'
Thread 1: EXC_BREAKPOINT (code=1, subcode=0x100c2204c)

Expected behavior

The compiler should detect this issue at compile-time and provide one of the following:

• Compile-time error indicating that T() cannot be called because T doesn't guarantee a default initializer is available.
• Compiler warning about potentially unsafe T() instantiation in generic contexts

The current behavior violates Swift's principle of compile-time safety by allowing code that will predictably crash at runtime. The constraint T: UIView should either guarantee that T() is safe to call, or the compiler should warn/error when T() is used without such guarantees.

Environment

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)

Additional information

No response

[hamishknight]

Note this isn't specific to generics, you can also reproduce with e.g:

import Foundation

class C: NSObject {
  init(x: Int) {}
}
(C.self as NSObject.Type).init()

Looks like Clang imported declarations are exempt from the "dynamic metatype can only invoke required initializers" rule since there's no way to explicitly mark an Obj-C initializer as required AFAIK, it's instead only applied for matching protocol requirements (e.g init(coder:)).

[slavapestov]

Yeah this is working as intended, because Objective-C doesn't have a notion of required initializer, and trying to enforce this in some way would break too much source code at this point.

This is also rdar://27513697.