[Need Compiler Expert] Is there a way to workaround 'Symbol not found' crash on iOS 15 (sim) for APIS available since iOS 13?

Using Swift
17

Checking the implementation of SwiftUI on iOS 15.5 :arrow_down:

struct SwiftUI._Velocity<A> where A: Equatable{
    var valuePerSecond: A
}

It already has a "Equtable" constraint on _Velocity's generic type. So the conformance seems reasonable for me.

I guess with iOS 16/17 SDK, Apple has moved extension CGSize: Swift.Equatable or CGSize def to somewhere else. And that's the problem here.

On iOS 15.5 SDK, CGSize is defined on CoreGraphics. So if the Equatable conformance is also defined on CoreGraphics, the conformance is not a RetroactiveConformance.

// CoreGraphics - CGGeometry.h
/* Sizes. */

struct CGSize {
    CGFloat width;
    CGFloat height;
};
typedef struct CG_BOXABLE CGSize CGSize;

However on iOS 17.0 SDK, CGSize is defined on CoreFoundation instead. Then the conformance would be a RetroactiveConformance.

// CoreGraphics - CGGeometry.h

#ifndef CF_DEFINES_CG_TYPES
/* Sizes. */

struct CGSize {
    CGFloat width;
    CGFloat height;
};
typedef struct CG_BOXABLE CGSize CGSize;
...
#endif /* CF_DEFINES_CG_TYPES */

// CoreFoundation - CFCGTypes.h
...
#define CF_DEFINES_CG_TYPES

struct CGSize {
    CGFloat width;
    CGFloat height;
};
typedef struct CF_BOXABLE CGSize CGSize;

I was wondering how Apple keeps such compatibility for other public binary. You need to find a public symbol in Apple's system framework which use CGSize as generic type conforming to Equatable type just like this one. Then we can continue the investigation. cc @DevAndArtist

If you can find another public one, two results:

  1. It works fine. Then we can check the magic Apple is using. And try to apply it to our case.
  2. It does not work and crash too. Since this a public symbol, then you can file a FB and ask for a fix for it. Hopefully we'll get a new Xcode/iOS SDK later to fix the
    compatibility issue.
Here are some of my irresponsible personal guesses.

I found that many types of CG on Linux are defined in the Foundation module of swift-corelibs-foundation defined using Swift. But on previous Darwin platform, they were defined on CoreGraphics module.

So I often need to use the following code to use CG type on my cross-platform package.

OpenSwiftUI/Sources/OpenSwiftUI/Layout/LayoutAdjustments/Edge/EdgeInsets.swift at 56d29ff98a989ae5604eb184b01c0256fe4b3660 · OpenSwiftUIProject/OpenSwiftUI · GitHub

With the launch of the new swift-foundation and Apple want to make it universal in all platforms Swift support, I guess Apple then decided to move these CG types to CoreFoundation which exist on Linux as an implementation detail.

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